Frog Its Reproduct 00 r Ugh

IheReproduction and Development

jffl^^^^^^^^^^^^^JEaE

Marine Biological Laboratory Library

Woods Hole, Mass.

*/^>«^v.

PreBented ty

The Blakiston CompanyPhiliidelphia

[D

The FROGIts Reproduction and Development

cry/- 16 IS

6.^

The FROGIts Reproduction and Development

By

ROBERTS RUGH, Ph.D.

Associate Professor of Radiology (Biology),

Department of Radiology, Columbia University

VMaMphxa • THE BLAKISTON COMPANY • loronio

1951

Copyright, 1951, by The Blakiston Company

This book is fully protected by

copyright, and no part of it, with

the exception of short quotations

for review, may be reproduced

without the written consent of

the publisher

PRINTED IN THE UNITED STATES OF AMERICA

AT THE COUNTRY LIFE PRESS, GARDEN CITY, N.Y.

Prefrerace

This book has been written for three reasons. First, since T. H. Mor-

gan's "Development of the Frog's Egg" (1897), there has been no

book written specifically on this subject. His was a most excellent

treatise, still remarkably accurate. Second, with the accelerated in-

terest in the experimental approach to the study of embryology,

stimulated by Spemann in Germany and Morgan and Harrison in

this country, and implemented by a host of their students, there

has been accumulated a large volume of information not available

in 1897. It is important that the description of normal embryology,

aided by this experimental approach, be brought up to date for the

frog. Third, with the discovery that the frog can be induced to

ovulate and provide living embryos at any time of the year, these

embryos have become one of the major test organisms in experimental

embryology. It is also being used increasingly in the related scientific

disciplines such as physiology, cytology, and genetics.

The author disclaims any fundamental originality in this book.

All of the previous investigators and authors touching on the normal

embryology of the frog, from whose works much information has

been gathered, have been listed in the Bibliography. It is they whohave done much of the "spade work" for this book and the student

is encouraged to refer to these original sources. However, the author

has described the normal development of the frog to more than 5,000

students during 19 years of teaching, as a result of which an intimate

personal knowledge of all phases of frog embryology has inevitably

accrued. Further, the author organized one of the first laboratory

courses for experimental embryology in which the major experimental

form was the frog egg and embryo. From these two major lines of

work a personal interpretation of the development of the frog egg and

embryo has developed, built on the broad structural foundation laid

by a host of other workers. The author is responsible, however, for

any novelty of interpretation.

The author has no intention of claiming any suggestion of final-

VI PREFACE

ity, in spite of a didactic presentation. Tlie objective presentation of

the truth, as far as it can be apprehended at the moment, is the

extent of one's responsibility. Accuracy should be the moral and

ethical responsibility of every author of a treatise on a scientific sub-

ject. To the best of the author's knowledge, the descriptive material

of this book is demonstrably accurate.

The author, as a teacher, has found it increasingly important that

there be a common language by which information can be imparted

to the student. This necessity is met in part by a Glossary of embryo-

logical terms, readily accessible to the student and rigidly adhered to

by the instructor. For this reason, the book breaks with tradition to

include a complete Glossary of some 750 words. A definition will

often clarify or crystallize a complicated and detailed description.

Therefore it is hoped that the student will increase his functional

vocabulary to the extent of the appended Glossary.

The author is also an enthusiastic advocate of the visual elucida-

tion of the oral description. Most illustrations are useful, some are

indispensable. Therefore a profusion of illustrations appears in this

book, most of which are original and based on direct observation of

the egg and embryo. In a few cases excellent illustrations of other

workers have been borrowed or slightly modified for inclusion in this

text.

The seed for this book was planted early in the author's mind while

he was being initiated into the field of embryology by Professor Robert

S. McEwen of Oberlin College. During a long period of incubation

the plan slowly matured. Experience, gained over the years in teach-

ing and learning from the response of interested students, helped to

bring into clearer focus the aims to be sought for in this book as a

teaching text. The execution of the task and the finished form pre-

sented here could not have been attained at this time but for the in-

valuable assistance given by the publishers and their staff. Thanks

are due to Miss Marie Wilson and to The Blakiston Company

—

especially to Mr. William B. McNett and Miss Gloria Green of the

Art Department who helped with the illustrations; to Mr. Willard

Shoener of the Production Department; and most particularly to Miss

Irene Claire Moore and Dr. James B. Lackey of the Editorial Staff.

Special acknowledgment is also given to the General Biological

Supply House of Chicago, whose illustration of the frog is used so

effectively on the title page. It remains for the professor and the

PREFACE Vll

student of embryology to evaluate the fruits of our common labors.

Embryology is a basic subdivision of biology. From it stem the

anatomy, the histology, and the physiology of the adult. To under-

stand it well is to aid in the comprehension of the other biological

disciplines.

To know one's self it is not sufficient to study the present-day

transiency. Such a study will be enhanced to the degree that it is sup-

ported by a knowledge of what preceded the present. To be a Jacques

Loeb and thus be qualified to make such a statement as "The most

uninteresting thing I know is the normal development of an egg," one

has first to know formal, basic, morphological embryology as thor-

oughly as it is possible to comprehend. It is only then that we can

appreciate and intelligendy apply the experimental method to the

problems of embryology. Possibly, were Loeb alive today, he would

be one of the first to admit that we know less about the normal de-

velopment of the frog than we thought we knew at the time of his

statement. Even for the ten-thousandth time, the formation of the

tension lines of the first cleavage furrow or the initial involution of the

gastrula are still among the most challenging of unsolved mysteries

both to the author and to his students.

The adult "organism as a whole" is, in part at least, an expression

of its earlier experiences as an embryo. We may even have to admit

that the ultimate personality begins its realization at the moment of

fertilization. Certainly, to by-pass a study of the development of the

most rapid, the most dynamic, the most plastic stage of one's entire

physical existence is to miss the sum and substance of life itself. It

is the author's firm belief that anyone who completely understands

the mechanism of normal embryonic development will, to a com-

parable degree, understand life at any level. Further, in understand-

ing the embryology of the frog completely, one would come very close

to an understanding of the basic principles of development of any

form whatsoever.

Roberts RughNew York City,

June 1950.

Contents

Preface v

Chapter One. Introduction l

The Period of Descriptive Embryology 4

The Period of Comparative Embryology 6

The Period of Cellular Embryology 7

The Period of Experimental Embryology 7

The Embryologist as a Scientist 9

Why the Embryology of the Frog? 10

Some General Concepts in Embryology 11

Biogenesis, 11; Biogenetic Law—The Law of Recapitulation, 11; Epi-

genesis vs. Preformationism, 12; Soma and Germ Plasm, 13; The GermLayer Concept, 14

The Normal Sequence of Events in Embryology 14

Chapter Two. General Introduction to the Embryology of the

Leopard Frog, Rana pipiens 17

Chapter Three. Reproductive System of the Adult Frog: Ranapipiens 31

The Male 31

Secondary Sexual Characters 31

Primary Sexual Characters 31

Reproductive Behavior 40

Accessory Structures 41

The Female 43

Secondary Sexual Characters 43

Primary Sexual Characters 43

Chapter Four. Fertilization of the Frog's Egg 73

Completion of Maturation of the Egg 77

Penetration and Copulation Paths of the Spermatozoon 78

Symmetry of the Egg, Zygote, and Future Embryo 82

Chapter Five. Cleavage 87

Chapter Six. Blastulation 93

Chapter Seven. Gastrulation 97

Significance of the Germ Layers 97

Origin of Fate Maps 98

ix

69920

X CONTENTS

Gastrulation as a Critical Stage in Development 101

Pre-gastrulation Stages 103

Definition of the Major Processes of Gastrulation 107

Gastrulation Proper 109

Chapter Eight. Neurulation and Early Organogeny 123

Neurulation 123

Early Organogeny 124

Surface Changes 124

Visceral Arches 131

Origin of the Proctodeum and Tail 132

Internal Changes 136

Chapter Nine. A Survey of the Major Developmental Changes

IN the Early Embryo 139

Chapter Ten, A Survey of the Later Embryo or Larva 155

Chapter Eleven. The Germ Layer Derivatives 161

The Ectoderm and Its Derivatives 161

The Brain 161

The Spinal Cord 167

The Peripheral Nervous System 169

The Organs of Special Sense 170

The Cranial Nerves 181

The Spinal Nerves 185

The Neural Crest Derivatives 188

The Stomodeum and Proctodeum 189

Chapter Twelve. The Endodermal Derivatives 193

The Mouth (Stomodeum, Jaws, Lips, Etc.) 193

The Foregut 194

The Midgut 205

The Hindgut 206

Chapter Thirteen. The Mesodermal Derivatives 209

The Epimere (Segmental or Vertebral Plate) 209

The Mesomere (Intermediate Cell Mass) 218

The Hypomere (Lateral Plate Mesoderm) 231

Appendix: Chronological Summary of Organ Anlagen Appear-

ance OF Rana pipiens 251

Glossary of Embryological Terms 261

Bibliography for Frog Embryology 311

Index 321

CHAPTER ONE

Introduction

The Period of Descriptive Embryology Epigenesis vs. PreformationismThe Period of Comparative Embryology Soma and Germ PlasmThe Period of Cellular Embryology The Germ Layer ConceptThe Period of Experimental Embryology The Normal Sequence of Events in

The Embryologist as a Scientist EmbryologyWhy the Embryology of the Frog? Cell Multiplication

Some General Concepts in Embryology Cell Differentiation (Specialization)

Biogenesis OrganogenyBiogenetic Law—The Law of Re- Growth

capitulation

Embryology is a study of early development from the fertilized egg

to tlie appearance of a definitive organism. The stage is generally

referred to as its embryonic stage. It is not so inclusive a term as

ontogeny, which refers to the entire life history of an organism from

the fertilized egg to old age and death. Since either the sperm or the

egg (of a zygote) can affect development, the study of embryology

might well begin with a study of the normal production and matura-

tion of the germ cells (gametes). It should include fertilization, cleav-

age, blastula and gastrula formation, histogenesis, organogenesis, and

the nervous and humoral integrations of these newly developed organs

into a harmoniously functioning organism. When development has

proceeded to the stage where the embryo can be recognized as an

organism, structurally similar to its parents, it no longer can be re-

garded as an embryo.

The unfertilized egg is an organism with unexpressed potentialities

derived from an ovary containing many essentially similar eggs. Its

basic pattern of development is maternally derived and is predeter-

mined in the ovary but its genetic complex is determined at the very

instant of fertilization. This predetermination is in no sense struc-

tural (i.e., one never has seen a tadpole in a frog's egg) but is none-

theless fixed by both the nuclear and the cytoplasmic influences of

the fertilized egg (or zygote). These influences are probably both

physical and chemical in nature.

No amount of environmental change can so alter the development

1

2 INTRODUCTION

of a Starfish zygote (i.e., fertilized egg) that it becomes anything but

a starfish, or change the potentiaHties of a fertilized frog egg into

anything but a frog. This means that the nucleus and the cytoplasm

of the fertilized egg together possess certain potentialities, as well as

certain limitations, in development. Within the range of those limita-

tions the potentialities will inevitably become expressed, in any

reasonably normal environment.

It is now quite clear that both the nucleus and the cytoplasm are

influential in development. The genetic influences are largely nuclear,

but cellular differentiation is cytoplasmic. With each of the cleavages

there is a synthesis of nuclear material (e.g., desoxyribose nucleic

acid) out of the cytoplasmic (ribonucleic acid), and cytoplasmic

from the yolk or other extrinsic food sources. Development is not

simply growth or increase in mass. It involves a constant synthesis or

the building-up of those elements so vital to the normal processes in

the development of the individual. Each stage is built upon the suc-

cessful completion of the preceding stage.

Comparative embryologists have found that there is a somewhat

Frog's egg and swollen jelly shortly after fertilization.

INTRODUCTION 3

similar pattern to the development of all forms. The earlier in develop-

ment that the comparison is made, the greater the similarity among

even widespread species. This may mean that all other possible types

of development have failed to survive in a harshly selective environ-

ment. Or it may suggest a common ancestry. In any case, all embryos

do begin with the fertilized egg. (With the rare exceptions of natural

parthenogenesis.) The activated egg immediately manifests certain

metabolic changes which may be correlated with the kinetic move-

ments leading toward the completion of maturation or toward the first

cleavage. Division results in a shifting of the nucleo-cytoplasmic vol-

ume relations from the unbalanced condition of the ovum to the more

stable ratio found in the somatic cells. Each division of the egg means

more nuclear material, less cytoplasmic material, and more rigid cells.

This rigidity, the adhesiveness of cells, and their activity together

cause the development of sheets of cells which soon cover (chick)

or surround (frog) a cavity, known as the blastocoel. As this sheet

of cells expands it becomes necessary for it to fold under (chick), to

push inward (frog), or to split into layers (mammal), and thereby

form the 2-layered gastrula. In most forms almost immediately the

third layer (mesoderm) develops between the two preceding layers,

epiblast and endoderm. After the derivation of the mesoderm from

the outer layer, the latter is then known as the ectoderm. All of this

occurs before the appearance of any discernible organs. There is

reason to believe that these sheets of cells have topographical rather

than functional significance and that parts of any one of them could

be exchanged experimentally with any other at this time without

seriously disrupting the normal development of the embryo.

At this point, however, the process of cell division becomes second-

ary to the process known as differentiation or specialization of cell

areas. This is the very beginning of organ formation. Many embryos

begin to develop transient (larval) organs which are replaced as the

more permanent organs appear and begin to function. Eventually

there is produced a highly complex but completely integrated organ-

ism which is able to ingest, digest, and assimilate food, and subsist

for itself independently of its parent organism. It is then no longer

considered an embryo.

This period of ontogeny is the subject matter of embryology. Weattempt not to answer the major biological question "why" but

rather to confine ourselves to a detailed description as to "how" an

4 INTRODUCTION

organism is produced from a fertilized egg. It is the purpose of this

book to describe "how" the frog develops from the fertilized frog's

egg, as we understand it. The facts of this book are made available

through the accumulated studies of many investigators, in this and

other countries.

There are those who are interested in the subject matter of embry-

ology solely because they were once embryos themselves, or because

they anticipate becoming partners in the further production of em-

bryos. The closer we can come to the understanding of the mecha-

nism of embryonic development of any single species, the closer will

we come to the understanding of the mechanism of life itself. The

processes of the embryo are certainly fundamental to all of life. Life

exists by virtue of successful embryonic development. There are few

things in the living world more absorbing or more challenging to

watch than the transformation of a single cell into a complex organ-

ism with its many organs of varied functions, all most efficiently in-

tegrated.

Embryology is not a new subject. It has a very rich heritage, based

upon the solid foundation of pure descriptive morphology. This is

followed by the comparison of the variations in development, leading

to the recent trend toward the physical and chemical analyses of

the developmental processes. In order that we do not lose sight of

this heritage, a brief survey is given in the following pages.

The Period of Descriptive Embryology

While organisms must have been reproducing and developing

since the beginning of life, knowledge of these processes seems to

have begun with Aristotle (384-322 B.C.), who first described the

development and reproduction of many kinds of organisms in his "DeGeneratione Animalium." Since he could not locate the small mam-malian egg, he considered its development to be the most advanced

of all animals. Below the mammal he placed the shark, whose young

develop within the body of the female but are born alive and often

with the yolk sac attached. Next, below the sharks, he placed the

reptiles and the birds whose eggs are complete in that they are pro-

vided with albumen and a shell. The lowest category of development

was that of the amphibia and fish, which had what he termed "in-

complete" eggs, referring, no doubt, to the method of cleavage.

Aristotle believed that development always proceeds from a simple

THE PERIOD OF DESCRIPTIVE EMBRYOLOGY 5

and formless beginning to a complex organization characteristic of

the adult. Basing this observation on his study of the hen's egg, he

laid the foundation for the modern concept of epigenesis or unfold-

ing development. This concept is opposed to the idea of structural

preformation of the embryo within the gamete, or germ cell.

William Harvey (1578-1657) long ago came to the conviction

that all animals arise from eggs: "Ex ovo omnis"; and later Virchow

(1821-1902) went even further to state that all cells are derived

from preexisting cells: "Omnis cellula e cellula." Flemming stated:

"Omne vivum ex nucleo" and later Huxley said, "Omne vivum ex

vivo." These concepts, taken for granted today, are basic in biology

and emphasize the fact that only through the process of reproduction

has the present population of organisms come into existence. Embry-

onic development is a prerequisite to survival of the species. There-

fore, only those organisms that survive their embryonic development

and achieve the stage of reproductive ability can carry the baton of

protoplasm from the previous to the future generations in the relay

race of life with time.

Fabricius (1537-1619) and Harvey were both limited in their

studies by the lack of the miscroscope but both of them presented re-

markable studies of early chick development. Malpighi (1628-94),

using the newly invented microscope, gave us two works in em-

bryology: "De Formatione Pulli in Ovo" and "De Ovo Incubato,"

which deal largely with the development of the chick from 24 hours

to hatching. Beginning at this stage he was misled to believe that all

the various parts of the embryo are preformed within the egg (since

chick embryos incubated for 24 hours exhibit most of the major organ

systems) and that the process of development was one simply of

growth and enlargement. This was the theory of "preformationism"

which is diametrically opposed to that of epigenesis or unfolding de-

velopment. This new concept of preformationism found a staunch

supporter in Swammerdam (1637-80). In 1675 Leeuwenhoek firmly

believed he discovered the human form sitting in a cramped position

in the head of the human spermatozoon. This led to the "spermist"

school of preformationists.

It was inevitable that, when parthenogenesis was discovered, the

spermists would have to retire in favor of the ovists (e.g.. Bonnet)

since eggs were known to develop into organisms without the aid of

spermatozoa. This led to the equally ridiculous concept of the "em-

INTRODUCTION

boitement" or "encasement" theory, which suggested that each egg

contains, in miniature, all the future generations to be developed

therefrom. Each generation was achieved by shedding the outermost

layer, like Chinese ivory boxes carved one within another. But pre-

formationism of either school was des-

tined to lose adherents as the microscope

became further refined. Caspar Friedrich

Wolff (1733-94) attacked the idea of pre-

formationism and supported epigenesis

on purely logical grounds, put forth in

his "Theoria Generationis." In 1786 he

published the most outstanding work in

the field of embryology prior to the works

of von Baer. It was entitled "De Forma-

tione Intestinorum" and in this treatise

Wolff showed that the intestine of the

chick was developed de novo (epigeneti-

cally) out of unformed materials.

The Period of Comparative Embryology

Up to about 1768 embryology was al-

most exclusively descriptive and morpho-

logical. It was inevitable that the second

phase in the history of embryology would

soon develop, and would be of a compar-Spermist's conception of the ative nature. This approach was stimu-

human figure in miniature j^^^^ ^ q^^-^^^ (1769-1832) and hiswithin the human sperm. (Re- , . .• . tJ r^ /^ Tj * • f emphasis on comparative anatomy. Indrawn after O. Hertwig, from ^ ^ ^

Hartsoeker: 1694.) 1824 Prevost and Dumas first saw cleav-

age or segmentation of an egg in reptiles.

In 1828 von Baer published his "Entwicklungsgeschichte der Tier"

and thereby became the founder of embryology as a science. He estab-

lished the germ layer doctrine, proposed a theory of recapitulation,

and made embryology truly comparative. From a study of the devel-

opment of various animals, von Baer arrived at four important con-

clusions, which are known collectively as the laws of von Baer. They

are as follows:

1 . "The more general characteristics of any large group of animals appear

in the embryo earlier than the more special characteristics."

THE PERIOD OF EXPERIMENTAL EMBRYOLOGY 7

2. "After the more general characteristics those that are less general arise

and so on until the most special characteristics appear."

3. "The embryo of any particular kind of animal grows more unlike the

forms of other species instead of passing through them."

4. "The embryo of a higher species may resemble the embryo of a lower

species but never the adult form of that species." (This latter state-

ment is the basis of the Biogenetic Law when it is properly inter-

preted.)

Following von Baer, Kowalevsky (1866) stated that all animals

pass through a gastrula stage. Haeckel (1874) proposed a Gastrea

Theory which suggested that the permanent gastrula, the adult

Coelenterata, might be the form from which all higher diploblastic

(gastrula) stages in their life history were derived.

The Period of Cellular Embryology

As the microscope was still further refined, and embryos could be

studied in greater detail, it was natural that a further subdivision of

the field of embryology would occur. In 1831 Robert Brown dis-

covered the nucleus; in 1838 Schleiden and Schwann founded the cell

theory; in 1841 Remak and Kolliker described cell division; in 1851

Newport observed the entrance of the spermatozoon into the frog's

egg; and in 1858 Virchow published his "Cellular Pathology." Later,

in 1878, Whitman and Mark initiated the study of cell lineage

("Maturation, Fecundation, and Segmentation of Limax") by which

the fate of certain early blastomeres of the embryo was traced from

their beginning. In 1882 Flemming discovered the longitudinal

splitting of chromosomes; and Sutton in 1901 gave us the basis for

the modern chromosomal theory of inheritance. The study of the

embryo was thus broken down into a study of its constituent cells;

then to the nucleus; and finally to the gene. From the morphological,

physiological, or genetic aspects, this field of cellular embryology is

still very active.

The Period of Experimental Embryology

During the latter part of the nineteenth century, investigators began

to alter the environment of embryos, and surgically and mechanically

to interfere with blastomeres and other parts of the developing em-

bryo. This new "experimental" approach required a prior knowledge

of morphological and comparative embryology. It was hoped that, by

8 INTRODUCTION

altering the physical and chemical conditions relating to the embryo,

the normal mechanism of development would be understood better

through an analysis of the embryonic adjustments to these altera-

tions. Before the turn of the century, the earlier workers in this field

were His, Roux, Weismann, Born, Driesch, and the Hertwigs (Oscar,

Richard, and Paula). Then came Morgan, Spemann, and Jacques

Loeb. Finally during the last several decades there has developed a

host of experimental embryologists, many of whom were inspired by

association with the above workers. Reference should be made briefly

to Adelmann, Baltzer, Bataillon, Bautzmann, Boell, Brachet,

Child, Conklin, Copenhaver, Dalcq, de Beer, Detwiler, Ekman, Fank-

hauser, Goerttler, Guyenot, Hadorn, Hamburger, Harrison, Herbst,

Holtfreter, Just, Korschelt, Lehmann, the Lewises, the Lillies, Man-

gold, Nicholas, Oppenheimer, Parmenter, Pasteels, Patten, Penners,

Rawles, Rotmann, Rudnick, Schleip, Schultz, Spratt, Swingle, Twitty,

Vogt, Weiss, Willier, E. B. Wilson, and a host of others.

A further refinement of this approach is in the direction of chemi-

cal embryology or a study of the chemistry of the developing embryo

and the raw materials from which it is formed. As Needham ( 1942)

says: "Today the interest has been shifted to the analysis of the funda-

mental morphogenetic stimuli which operate in embryonic life." Such

stimuli as these may well be of an ultra-chemical or ultra-physical

nature.

Embryology as a division of science has gone through a period of

its own development from the purely descriptive phase to the present-

day biochemical and biophysical analysis of development. However,

each generation must recapitulate this sequence; therefore each

student must begin with the foundation of basic, morphological em-

bryology before he can expect to comprehend the possibilities in the

superstructure of the experimental approach.

It is the function of this particular book to provide the student with

the foundational information relative to one genus, namely the com-

mon frog. This will introduce him to the major aspects of embryonic

development and at the same time give him a factual foundation upon

which he may later make his contribution in one of the fields of

embryology. Where it will not confuse, but might clarify the develop-

mental process, reference will be made to specific findings in the field

of experimental embryology.

THE EMBRYOLOGIST AS A SCIENTIST 9

The Embryologist as a Scientist

There are certain characteristics necessary for success and satis-

faction in pursuing the study of embryology, or, in fact, any science.

Some of these may be inherent, but it is more reaHstic to assume that

they can be developed.

First: One must have the completely open mind characteristic of

any true scientist. A student must come to any science without bias,

without preconceived or prejudiced concepts. He must be willing to

say: "Show me, give me proof, and then Fll believe." Science is a body

of knowledge, accumulated through generations by fallible human be-

ings. It is therefore as reliable as human experience but it is subject to

change with knowledge gained through further human experience.

Basically this body of demonstrated fact can be accepted at its face

value as a foundation upon which to build. It is a heritage of genera-

tions of trial and error, of observation, experimentation, and verifica-

tion. But the scientific attitude presumes that there is still much to

learn, and some ideas to be revised. The scientist maintains an open

mind, eager to be shown and willing to accept demonstrable fact.

Second: The student of embryology must have the ability to visu-

alize dynamic changes in a three-dimensional field. Most embryos are

not naturally transparent and it is difficult to observe directly what is

Post-erior

FRONTAL SECTION -LATERAL VIEW FRONTAL SECTION- DORSAL VIEW

Dorsal

TRANSVERSE SERIAL SECTIONS - LATERAL VIEW SAGITTAL SECTION - LATERAL VIEW

Planes in which the embryo may be cut or sectioned.

10 INTRODUCTION

transpiring within. It is necessary, therefore, to study such embryos

after they have been preserved, sectioned, and stained. Nevertheless,

the emphasis in embryology is on the dynamic changes that occur, on

derivations, and on the end organs of the developmental processes.

One is not interested in a single frame of a moving picture nor is it

sufficient to describe all of the structures in a single section of an em-

bryo. The student of embryology is interested in the composite picture

presented by a succession of individual frames (sections) which are

re-assembled in his mind into a composite, three-dimensional whole.

It is necessary, in dynamic embryology, to reconstruct in the mind the

inner processes of development which proceed from the single-celled

egg to the multicellular organism functioning as a whole. The embryo

also must be regarded in the light of its future potentialities. While

parts of the embryo are isolated for detailed study, the student must of

necessity re-assemble those parts into a constantly changing three-

dimensional whole. The embryo is not static in any sense—it is

dynamic.

Third: The student of embryology must have an intelligently directed

imagination, one based upon a foundation of scientific knowledge. He

must be rigidly loyal to demonstrable fact, but his mind must project

him beyond those facts. It is through men with intelligent imagina-

tion that there have been such remarkable advances in the super-

structure of embryology. Coupled with a healthy curiosity, such a

characteristic is causing men constantly to add facts, which withstand

critical investigation, to our ever-increasing body of knowledge.

Why the Embryology of the Frog?

It is the contention of the author that the student who understands

thoroughly the development of one species will have the foundation

for the understanding of the basic embryology of all species. This does

not imply similarity in development to the extent that there is no room

for comparative embryology of such forms as Amphioxus, fish, frog,

reptile, bird, and mammal. But the method of study, the language

used, and the fundamental processes are sufficiently alike so that a

thorough understanding of one form will aid materially in the under-

standing of the embryology of any other form. It is too much to cata-

pult a student into the midst of comparative or experimental em-

bryology and expect him to acquire any coherent conception of normal

development.

SOME GENERAL CONCEPTS IN EMBRYOLOGY 11

The normal embryology of the frog is well understood, and there is

no better form through which to introduce the student to the science

of embryology. The frog is a representative vertebrate, having both

an aquatic and a terrestrial existence during its development. This

metamorphosis requires, for instance, several major changes in the

respiratory and excretory systems during development. The embry-

ology of the frog illustrates the sequence of developmental events

(organogeny) of all higher vertebrates. Finally, the living embryos of

the frog are now available at any season of the year and students can

observe directly the day-to-day changes that occur from fertilization to

metamorphosis.

Some General Concepts in Embryology

Biogenesis

There is no substantiated evidence of spontaneous generation of

life, although, as a scientist, one cannot deny its possibility. All life

does come from preexisting life. In embryology we are more specific

and state that all organisms come from eggs. This statement is cer-

tainly true of all Vertebrates and of the vast majority of animals. How-ever, it cannot be said of the single-celled Protozoa or of some of the

lower Invertebrates which reproduce by binary fission, spore forma-

tion, or budding.

All protoplasm in existence today is believed to be descended from

preexisting protoplasm and it is therefore related by a continuous line

from the original protoplasmic mass, whenever that came into ex-

istence. Further, sexual organisms living today are each descendants

of a continuous line of ancestors not one of which failed to reach the

period of sexual productivity. So, the mere existence of an organism

today is testimony of its basic relationship to all protoplasm and to its

inherited tendency toward relative longevity. At present one cannot

conceive of life originating in any manner but from life itself. All

theories are, of necessity, philosophic speculation.

Biogenetic Law—The Law of Recapitulation

The original laws of von Baer are very clearly stated, but Haeckel

and others have misinterpreted or elaborated on them so that a con-

fusion about these concepts has arisen in the popular mind. Von Baer

clearly emphasized the fundamental similarity of certain early stages

12 INTRODUCTION

of embryos of various forms. He never suggested that embryos of

higher forms recapitulated the adult stages of their ancestors; that manas an embryo went through such stages as those of the adult fish,

amphibian, reptile, bird, and finally anthropoids and man. The simi-

larities referred to were in the early developmental stages, and most

definitely not in the adults.

As one studies comparative embryology it becomes clear that there

is a basic similarity in development, and that the brain, nerves, aortic

arches, and metameric kidney units (for instance) develop in a some-

what similar manner from the fish to man. Proponents of the theory

of evolution have emphasized this as evidence in support of their

theory. It is, however, possible that this similarity is due to a highly

selective environment which has eliminated those types of develop-

ment which have digressed from a certain basic pattern. Put another

way, the type of development we now know was suited to survive in

the environments available. Three primary germ layers may have

proved to be more efficient (i.e., to have better survival value) than

two or four, and so the gradual transition from pro- through meso-

and finally to the meta-nephros may be a necessary corollary of the

slow developmental process. Von Baer's laws may simply represent

one method of expressing a fundamental law of nature (i.e., survival

values) rather than a phylogenetic relationship.

There are many specific instances in development which could be

used to refute the Law of Recapitulation as it is often stated, but which

would in no way refute von Baer's "Biogenetic Law." One example

may be cited: there is no evidence among the lower forms of any antici-

pation of the development of the amnion and the chorion of the

amniotes. The Biogenetic Law was derived from a study of compara-

tive embryology, and possibly it has significance in the understanding

of the mechanism of evolution.

Epigenesis vs. Preformationism

The historical sequence from preformationists to spermists and

then ovists was very natural. The refinement of optical equipment

dispelled these earlier concepts and the pendulum swung to the opposite

extreme where embryologists believed that nothing was preformed

and that development was entirely epigenetic. As often occurs, the

pendulum has swung back to an intermediate position today.

No one has ever seen any preformed structure of the organism in

SOME GENERAL CONCEPTS IN EMBRYOLOGY 13

any germ cell. Nevertheless, one can bring together in the same re-

ceptacle the fertilized eggs of closely and distantly related forms and

yet their individual development is unaffected. A normally fertilized

egg of the starfish will always develop into a starfish, a frog's egg into

a frog, a guinea pig egg into a guinea pig. The environment of the

fertilized egg has never been known to cause the transmutation of

species. Further, we cannot see the stripes on a 2-cell mackerel egg,

or the green and yellow spots on the 2-cell frog's egg, or the colorful

plumage on a peacock blastoderm, or the brown eyes of the human

optic vesicle. Nevertheless these intrinsic genetic potentialities are defi-

nitely preformed and inevitable in development under normal en-

vironmental circumstances. Development is epigenetic to the extent

that one cannot see the formed structures within the egg, or the sperm,

or zygote. However, the organism is preformed chemically (geneti-

cally) to the extent that its specific type of development is inevitable

under a given set of circumstances. Our modern interpretation is there-

fore intermediate between that of preformationism and epigenesis.

There is unfolding development within certain preformed limits which

must be chemical and/or physical, but not visibly morphological.

Soma and Germ Plasm

It is definite that in some forms there is an early segregation of

embryonic cells so that some will give rise to somatic (body) tissues

while others will give rise to germ (reproductive) tissues. This was

the contention of Weismann and many others about 1900. The germi-

nal cells seem to come from the extra-embryonic regions of manyvertebrates and become differentiated at an early stage of develop-

ment. Whether or not these migrating pre-germ cells are the precursors

of the functional gametes has not been determined conclusively.

Nevertheless, the germ plasm, once segregated, is immune to damage

by the somatoplasm. This has been illustrated many times by menwho have incurred injuries and yet who have subsequently produced

normal offspring. The germ plasm is therefore functionally isolated,

segregated in the adult. Further, x-ray damage to the germ plasm does

not show up in the somatoplasm at least until the next generation.

Germ plasm can and does give rise to somatoplasm during normal

embryonic development. There is little, if any, evidence that somato-

plasm, once formed, can give rise to germ plasm. When the fully

formed gonads of higher animals are removed in their entirety, there

14 INTRODUCTION

seems to be no regeneration of germinal tissues from the remaining

somatic tissues.

However, modern embryologists are rather reluctant to believe that

these two types of protoplasm are as fundamentally segregated from

each other as was once believed. Regeneration is a characteristic of

lower forms. In these cases it certainly involves a redevelopment of

germinal tissue (e.g., Planaria regeneration of a total organism from

one-sixth part completely devoid of germinal tissue). The lack of

regenerative powers in general among the higher forms may be the

intervening factor in the distinction between somatoplasm and germ

plasm, and the lack of interchange or regeneration between them.

The Germ Layer Concept

In 1817 Pander first identified the three primary germ layers in the

chick embryo, and since then all metazoa above the Coelenterata have

been proved to be triploblastic, or tri-dermic. The order of develop-

ment is always ectoderm, endoderm, and then mesoderm, so that the

most advanced forms in the phylogenetic scale are those possessing

mesoderm.

We tend to forget that these germ layer distinctions are for human

convenience and that the morphogenetic potentialities are relatively

unaware of such distinctions. One can exchange presumptive regions

of the blastula so that areas normally destined to become mesoderm

may remain in a superficial position to function as ectoderm, or be-

come endoderm. The exchanges may be made in any direction in the

early stages. The embryo as a whole may develop perfectly normally,

with exchanged presumptive germ layer areas.

When we remember that all tissues arise from cells having the

same origin (i.e., from the zygote), and that mitosis ensures similar

qualitative and quantitative inheritance, then the presumed distinc-

tions of the three primary germ layers seem to dissolve. It is only dur-

ing the later phases of development (i.e., during differentiation) that

the totipotent genetic capabilities of the cells become delimited, and we

have the appearance of structurally and functionally different cell

and tissue types.

The Normal Sequence of Events in Embryology

Cell Multiplication.

From the single cell (the fertilized egg or zygote) are derived the

many millions of cells which comprise the organism. This is done by

THE NORMAL SEQUENCE OF EVENTS IN EMBRYOLOGY 15

the process of almost continual mitosis or cell division involving syn-

thesis and multiplication of nuclear materials. This continues through-

out the life of the organism but it is at its greatest rate during em-

bryonic development.

Cell Differentiation (Specialization).

After gastrulation the various cell areas, under new morphogenetic

influences, continue to produce more cells. However, some of these

cells begin to lose some of their potentialities and then to express cer-

tain specific characteristics so that they come to be recognized as cells

or tissues of certain types. Generally, after this process of differentia-

tion has occurred, there is never any reversion to the primitive or

embryonic condition. Differentiation is cytoplasmic and generally

irreversible.

Organogeny.

The formative tissues become organized into organ systems which

acquire specific functions. The embryo then begins to depend upon

these various newly formed organs for certain functions which are in-

creasingly important for the maintenance and integration of the

organism as a whole.

Growth.

This phenomenon involves the ability to take in water and food

and to increase the total mass through the synthesis of protoplasm.

Growth may appear to be quite uniform in the beginning. However,

as the organs begin to develop there is a mosaic of growth activity so

that various organ systems show peaks of growth activity at different

stages in embryonic development. It may be said that embryonic de-

velopment is completed when differentiation and organogeny have

been fully achieved.

CHAPTER TWO

General Introduction to tne Emnryolo^y

or tlie Leopard Fro^, Rana pipiens

The embryology of most of the Anura (frogs and toads) is essen-

tially the same. However, since the leopard frog, Rana pipiens, is so

abundant and is most generally used for embryologicai, physiological,

and morphological studies, the following description will be specifically

of this form. Where there are differences in closely related forms that

may be used in embryology, those differences will be indicated in the

text.

Rana pipiens (Schreiber) has a widespread distribution over entire

North America. It hibernates in marshes or pools and seems to prefer

the swampy marshlands for breeding in the spring. It may be found in

hay fields where there are many insects, but it remains close to a supply

of relatively calm water.

When these frogs are sexually mature they measure from 60 to

110 millimeters from snout to anus, the female being about 10 mm.longer than the male of the same stage of maturity. The body is slender

and the skin is smooth and slimy, due to a mucous secretion of the

integument. The general color is green, except when the animal is

freshly captured from hibernation. At this time the chromatophores

are contracted by the cold and the frogs have a uniform light brown

color. Extending backward from the eyes are a pair of light colored

elevations known as the dorsal plicae, between which are two or three

rows of irregularly placed dark spots. Each spot has a light (gen-

erally yellow) border. On the sides of the body these spots are usually

smaller and more numerous, and on the legs they are elongated to ap-

pear as bands. Occasionally one may find a dark spot on the tip of

each eyelid. Sometimes a light colored line, bordered below by a dark

stripe, occurs along the jaw and extends posteriorly below the tympanic

membrane and above the forearm. The ventral aspect of the body is

always shiny and white.

The period of germ cell formation occupies much of the long in-

17

18 INTRODUCTION TO THE EMBRYOLOGY OF THE LEOPARD FROG

Ovary

Male Female

The leopard frog, Rana pipiens (photographs by C. Railey).

INTRODUCTION TO THE EMBRYOLOGY OF THE LEOPARD FROG 1 9

terval between the annual spring breeding seasons. Breeding occurs

normally between the first of April and the latter part of May,

depending upon the latitude and upon variations in the tempera-

ture. Therefore Rana pipiens breeds for about two months, as the

temperature rises, from Texas to Canada. In any case, breeding gen-

erally occurs before the frogs have had an adequate opportunity to

secure food. This means that they must call upon what reserves of fat

and glycogen they did not consume during the extended period of

hibernation. The interval from egg-laying to metamorphosis of the

tadpole is about 75 to 90 days, this phase of development being com-

pleted well before the next hibernation period.

Eggs generally are shed early in the morning, and Rana pipiens will

lay from 2,000 to 3,000 of them. The bullfrog, Rana catesbiana, has

been known to lay as many as 20,000 eggs. These are laid in heavy

vegetation to which their jelly coverings make them adherent. Eggs

are found usually in shady places, floating near the surface in rather

shallow water.

Fertilization takes place during amplexus, the term for sexual em-

brace of female by male, as the eggs are laid (oviposition) by the

female. The cleavage rate depends upon the temperature of the en-

vironment and there may be a lag of from IVi to 12 hours between

Animal hemisphere

Gray crescent

Vegetalhemisphere

Stoge 2. 1 hr post-

fertilization. Right

side view.

Stage 3. First cleavage Stage 4. Second Stage 5. Third

at 3.5 hrs. Posterior cleavage at 4.5 hrs. cleavage at 5.4

view. Right side view. hrs. 8 cells.

Stage 7. Fifth cleavage.

32 cells at 7 hrs.

Stage 10. Earliest

involution of dorsal

lip of 26 hrs. Pos -

terior view.

Stage II Extension

of dorsal to lateral

lips at 34 hrs.

Posterior view.

Stage 12 Completelip involution, en-circling yolk at

42 hrs.

Early development of the frog's egg.


the fertilization of the egg and the appearance of the first cleavage

furrow. Often frogs are misled by an early thaw and proceed to shed

and fertilize their eggs, and then the pond freezes over. Such eggs

usually can withstand a brief (1 to 2 days) freezing without serious

effects. Once cleavage has begun, it must proceed (within certain

limits of speed) until the egg is divided into progressively smaller and

smaller units, first known as blastomeres and later as cells. The first

cleavages are quite regular. Since the egg has so much yolk the

division of parts of the egg becomes very irregular after about the 32-

cell stage. The cleavage planes in the early stages may be altered by

unequal pressures applied to any egg within a clump of eggs.

The blastula develops an eccentric cavity because the animal pole

cells are so different from the large yolk cells of the vegetal hemisphere.

However, the end of the blastula stage is the end of the cleavage stage,

although cell division goes on throughout the life of the embryo, the

larva, and finally the frog.

The gastrula is an embryo having two primary germ layers, the

epiblast (presumptive ectoderm and mesoderm) and the endoderm.

Gastrulation in the frog.

The second layer is continuous with the first and develops by integrated

movements of sheets of cells. There results the formation of a new

and second cavity known as the gastrocoel, or archenteron, which is

the primary embryonic gut cavity. The opening into this cavity is the

blastopore, and is located in the approximate region of the posterior

end of the gut cavity, or the region of the anus.

The process of gastrulation in the frog is completed by providing

INTRODUCTION TO THE EMBRYOLOGY OF THE LEOPARD FROG 21

also the third germ layer, or the mesoderm, and the notochord, which

come from the epiblast. The notochord is the axis around which the

vertebral column will be built. The mesoderm will give rise to the

bulk of the skeleton and muscle, to the entire circulatory system, and

to the epithelium which lines the body cavity.

Shortly after gastrulation the embryo elongates and develops a dor-

sal thickening known as the medullary plate. This thickened ectoderm

Left

r^R R

Posterior

No 13. Early neurula. Dorsal No.l4. Neural fold stage. No. 15. Closing neural fold

view Medullary plate stage Dorsal view. Dorsal view.

Brain region

Body Toil Gill onlage

No 16. Early tail bud.

Dorsal view.

Sucker

No, 17. Earliest musculor response.Lateral view.

External gills

Stoge 20, 6mm I40hrsGill circulation andhatching.

External gills

Stage 25 llmm 284hrs External gills

Spiracie absorbed. Left side to show opercular

fold and spiracle.

Early development of the frog embryo. (Top) Development of the axial central

nervous system. (Bottom) Development of the external gills and operculum.


will give rise to the entire central nervous system. It closes over dorsally

to form the neural and the brain cavities. These continuous cavities

are later almost obliterated by the growth and expansion of their walls

to form abundant nervous tissue. Extensions of this central nervous

system grow out into all parts of the body and all organs as nerves.

Dorsal lip

/"^


achieved, the organ systems are not all developed to the adult form.

The external gills of the tadpole function for a short time and then

are replaced by internal gills. The external gills are covered by an

operculum or posterior growth of the hyoid arch, with but a small pore

or spiracle remaining on the left side of the head. This is the only

channel of egress for water from the pharynx and out over the internal

gills within the branchial chamber.

From the first day of hatching, when the total water content of the

tadpole is about 56 per cent, there is a very rapid rise in the water con-

tent until the fifteenth day after hatching, when it reaches the maxi-

-Toil bud

Stage 18

4mm.

Stomodeal cleft

-Gill bud

Olfactory pit

Stage 19

5mm.

Mouth

Externalgills

Opercularfold

Stage 239 mm.

Mouth

Oralsucker

-

Stage 206mm.

Stage 21

7mm.

-Mouth

Stage 228 mm.

Remnant,oral sucker

Spiracle

Development and absorption of the external gills of the frog larva.


mum point of 96 per cent water (Davenport, 1899, Proc. Bost. Soc.

Nat. Hist.) This imbibition accounts for the apparent acceleration of

growth of the newly hatched tadpole, but it is not related to an in-

crease in total mass since there is no immediate intake of food. The

growth rate is affected by various environmental factors such as space,

Metamorphosis of the frog, Rana catesbiana. {Reading from left to right, top

and bottom) : Tadpole; tadpole with hind legs only; tadpole with two pairs of

legs; tadpole with disappearing tail, ready to emerge from water to land; im-

mature terrestrial frog; mature frog.


heat, available oxygen, and pressure. The tadpole soon begins vora-

cious feeding on a vegetarian diet.

Shortly after hatching, finger-like external gills develop rapidly on

the posterior sides of the head and these constitute the only respira-

tory organs. Simultaneously with the opening of the mouth a series of

visceral clefts (gill slits) develop as perforations in the pharyngeal

wall, and their walls become folded to form internal gills. The ex-

ternal gills gradually lose their function in favor of the internal gills.

They then become covered over by a posterior growth of tissue known

as the operculum. There remains but a single excurrent pore, the

spiracle, on the left side at the posterior margin of the operculum.

There are but few changes in the respiratory system from this stage

until metamorphosis begins at about IVi months. The internal gills

lose their function in favor of lungs at metamorphosis and this allows

the aquatic tadpole to become a terrestrial frog. When the tadpole be-

gins to develop its lungs it frequently comes to the surface for air.

The forelimbs begin to grow through the operculum, and, about 2^/^

months after the eggs are fertilized, the hind legs begin to emerge and

the tadpole is ready for the critical respiratory and excretory changes

that accompany metamorphosis.

Metamorphosis in the leopard frog, Rana pipiens, occurs in from

75 to 90 days after the egg is fertilized, generally in the early fall and

at a time when the food becomes scarce and the cool weather is im-

pending. Metamorphosis is one of the most critical stages in frog de-

velopment, involving drastic changes in structure and in function of

the various parts of the body. The tadpole ceases to feed; loses its outer

skin, horny jaws, and frilled lips; the mouth changes from a small oval

suctorial organ to a wide slit and is provided with an enlarged tongue;

the eyes become enlarged; the forelimbs emerge; the abdomen shrinks;

the intestine shortens and changes histologically while the stomach and

liver enlarge; the diet changes from an herbivorous to a carnivorous

one; the lungs become the major respiratory organs with the moist

skin aiding; the mesonephros assumes greater function; the tail re-

gresses; sex differentiation begins; and the tadpole crawls out of the

water as a frog.

We may now summarize the steps in the development of the frog as

follows:

1

.

Fertilization of the egg

2. Formation of the gray crescent due to pigment migration


3. Early cleavage

4. Blastula stage—coeloblastula (see Glossary) with eccentric blastocoel

5. Gastrulation

Early—crescent-shaped dorsal Up

Middle—semi-circular blastoporal lip

Late—circular blastoporal lip

6. Neurulation

Early—medullary plate

Middle—neural folds converging

Late—neural tube formed and ciliation of embryo7. Tail bud stage—early organogeny

8. Muscular response to tactile stimulation

9. Early heart beat, development of gill buds

10. Hatching and gill circulation

1 1

.

Mouth opens and cornea becomes transparent

12. Tail fin circulation established

13. Degeneration of external gills, formation of operculum, development of

embryonic teeth

14. Opercular fold over branchial chamber except for spiracle; internal gills

15. Prolonged larval stage with refinement of organs

16. Development of hindlimbs, internal development of forelimbs in oper-

cular cavity

17. Projection of forelimbs through operculum, left side first

18. Absorption of the tail and reduction in size of the gut

19. Metamorphosis complete, emergence from water as miniature, air-

breathing frog

The rate of development of the egg and embryo will depend upon

the temperature at which they are kept. The approximate schedule of

development at two different temperatures is given below.

Stage At 18

Fertilization

Gray crescent 1

Rotation li/^

Two cells 3V2

Four cells 4V2

Eight cells 5V^

Blastula 18

Gastrula 34

Yolk plug 42

Neural plate 50

Neural folds 62

Ciliary movement 67

Neural tube 72

Tail bud 84

= c.

Stage Number

Age-Hours AT I6°C

UNFERTILIZED

Staqe Number

Age -Hours at I6X

7.5

32-CELL

Stage Number

Age- Hours AT l&'C

13 50

NEURAL PLATE

8 14- 62

GRAY CRESCENT MID-CLEAVAGE NEURAL FOLDS

3.5 915 61

TWO- CELL LATE CLEAVAGEROTATION

4.5 10 26

FOUR- CELL DORSAL LIP16 12

5.1

6.5

II 34NEURAL TUBE

EIGHT- CELL MID-OASTRULA

42

SIXTEEN-CELL

'1 64

UTE GASTRULA TAIL BUD

(From "Stages in the Normal Development of Rana pipiens," by Waldo Shum-

way. Reprinted from Anat. Rec, 78, No. 2, October 1940.)

27

Stage Numbeir

d

9

Age. in Hours at Id" Centigrade

96

16

Length in Millimeters

4

MUSCULAR RESPONSE.

ME.ART BEAT

20 140 6

GILL CIRCULATION MATCHING

162 1

MOUTH OPEN CORNEA TRANSPARENT

22 192 5

TAIL FIN CIRCULATION

(From "Stages in the Normal Development of Rana pipiens." by Waldo Shum-way. Reprinted from Anat. Rec, 78, No. 2, October 1940.)

28

Stage Number

23

Age. in Hours at IS** Centigrade

216

24240

Length in Millimeters

OPERCULAR rOLD TE.E.TM

OPERCULUM CLOSED ON RIGHT

25 284

OPERCULUM COMPLETE.

(From "Stages in the Normal Development of Rana pipiens," by Waldo Shum-

way. Reprinted from Anat. Rec, 78, No. 2, October 1940.)

29


Stage Atl8°C. At25°C.

Muscular movement 96 hrs 76 hrs.

Heart beat 5 days 4 days

Gill circulation 6 " 5

Tail fin circulation 8 " 6V^

Internal gills, operculum 9 " IV2

Operculum complete 12 " 10

Metamorphosis 3 mos IVi mos.

Those who wish to carry the tadpoles through to later development,

and even through metamorphosis into frogs, must begin to feed them

at about the time the external gills appear. The food consists of small

bits of green lettuce or spinach leaves, washed thoroughly and wilted

in warm water. The water in the finger bowls, or larger tanks, must be

cleaned frequently to remove debris and fecal matter and to prevent

bacterial growth. If the tadpoles are not crowded they will grow faster.

After about 10 days the numbers should be reduced to about 5 tad-

poles per finger bowl of 50 cc. of water. After metamorphosis, the

young frogs must be fed small living worms (Enchytrea) or forced-fed

small pieces of fresh liver or worms.

CHAPTER THREE

Reproductive System or tne Adult Fro^:

Jxana pipiens

The MaleSecondary Sexual Characters

Primary Sexual Characters

The Testes

Spermatogenesis

Reproductive Behavior

Accessory Structures

The FemaleSecondary Sexual Characters


The Ovaries

The Body Cavity and the Oviducts

Oogenesis—Maturation of the Egg

The Male

Secondary Sexual Characters

The mature male frog is generally smaller than the female, ranging

from 60 to 110 mm. in length from snout to anus. The identifying

features which distinguish it from the female are a darkened thumb

pad which changes thickness and color intensity as the breeding

season approaches; a distinct low, guttural croaking sound with the

accompanying swelling by air of the lateral vocal sacs located between

the tympanum and the forearm; a more slender and streamlined body

than that of the female; and the absence of coelomic cilia except in

the peritoneal funnels on the ventral face of the kidneys. Males of

many species carry additional features such as brilliant colors on the

ventral aspects of the legs {R. sylvatica), black chin {B. fowleri), or

the size and color of the tympanic membrane.


The Testes.

The testes of the frog are paired and internal organs and are sus-

pended to the dorsally placed kidneys by a double fold of peritoneum

known as the mesorchium. This mesentery surrounds each testis and is

continuous with the peritoneal epithelium which covers the ventral

face of each kidney and lines the entire body cavity.

31

32 REPRODUCTIVE SYSTEM OF THE ADULT FROG

The testes are whitish and ovoid bodies lying ventral to and near

the anterior end of each kidney. The vasa efferentia, ducts from the

testes, pass between the folds of the mesorchium and into the mesial

margin of the adjacent kidney. During the breeding season, or after

slight compression of the testis of the hibernating frog, these ducts be-

come the more apparent due to the presence in them of whitish masses

of spermatozoa in suspension. The ducts are very small in diameter,

tough walled, and interbranching. They are lined with closely packed

cuboidal cells. Each duct is connected directly with a number (8 to

12) of Malpighian corpuscles of the kidneys, by way of the Bow-

man's capsules. These connections are permanent so that many of

the anterior uriniferous tubules of the frog kidney will contain sperma-

tozoa during the breeding season. The presence of spermatozoa in

the kidney also can be achieved artificially by injecting the male

frog with the anterior pituitary sex-stimulating hormone. Since these

anterior Malpighian corpuscles carry both spermatozoa (during the

breeding season) and excretory fluids (at all times), they are truly

urogenital ducts having a dual function. This situation does not hold

for higher vertebrates.

The spermatozoa are produced in subdivisions of the testes known

as seminiferous tubules. These are closely packed, oval-shaped sacs,

which are separated from each other by thin partitions (septula) of

supporting (connective) tissue known as interstitial tissue. This tissue

presumably has some endocrine function. The thickness of this

tissue is much reduced immediately after breeding or pituitary stimu-

lation. The interstitial tissue is continuous with the covering of the

testes known as the tunica albuginea, and the whole testis is enclosed

in the thin peritoneal epithelium.

Spermatogenesis.

Shortly after the normal breeding season in the spring for Rana

pipiens, the spermatogonium, which has ceased all mitotic activity,

enters upon a period of rest but not inactivity. During this period the

nucleus passes through a sequence of complex changes which repre-

sent an extended prophase. This is in anticipation of the two matura-

tion divisions that finally produce the haploid spermatid which

metamorphoses into a spermatozoon.

The nucleus of the spermatogonium contains chromatin which ap-

pears as relatively coarse lumps distributed widely over an achro-

THE MALE 33

Primordiol germ cell - 2N

@1

SPERMATOGENESIS

(@)@(@)@(®)@(@)(@)

Primary oocyte {2N)(Mitotic=Equationcl

division)

First polar body

(Meiotic •Reductionol

division)

®N-Secondary spermatocyte (2N/ (Meiotic = Reductional division)

d) j-Spermatid (N)

(Metamorphosis)

Spermatozoon (N)

Frog's egg remains at metoptiase

of second maturation division until

it IS fertilized or dies

The maturation process. Schematized drawings. The post-reductional division

is shown. In many forms the first division is reductional and the second is

equational. The end result is the haploid gamete, in either instance.

matic reticulum. Both the cytoplasm and the nucleus grow and the

chromatic granules become finely divided and arranged into contigu-

ous rows, bound by an achromatic thread, together known as chromo-

somes. This is the leptotene stage of spermatogenesis. Shortly the

chromosomes become arranged in pairs which converge toward that

side of the nucleus where the centrosome is found. The opposite ends

of the paired chromosomes merge into the general reticulum. This

is the synaptene stage. The chromatin granules become telescoped

together on the filaments so that the aggregated granules, known as

chromosomes, appear much shorter and thicker. Pairs of chromo-

somes become intertwined and the loose terminal ends become coiled

and tangled together. This is the contraction or synizesis stage. Then

the members of the various pairs become laterally (parabiotically)

fused. While there is no actual reduction in total chromatin, there

is a temporary and an apparent (but not real) reduction in the total

number of chromosomes to the haploid condition, because of this fu-

sion. There is no actual reduction in the total amount of chromatin


Prophases of the heterotype division in the male Axolotl. (1) Nucleus of

spermogonium or young spermocyte. (2) Early leptotene. (3) Transition to

synaptene. (4) Synaptene with the double filaments converging toward the

centrosome. (5) Contraction figure. (6, 7) Pachytene. (8) Early diplotene.

(9) Later diplotene. (10) The heterotypic double chromosomes; the nuclear

membrane is disappearing. (Courtesy, Jenkinson: "Vertebrate Embryology,"

Oxford, The Clarendon Press.)

material, nor is there any permanent reduction at this stage in the

number of chromosomes. Their identity is lost only temporarily. This

is known as the pachytene stage. The members of each pair then

separate again. It must be remembered, however, that (a) the separa-

tion need not be along the original line of fusion and that (b) an

exchange of homologous sections of the chromosomes may occur

without any cytological evidence. In any case, the diploid number

of chromosomes reappears and this is then known as the diplotene

stage.

THE MALE 35

During these changes in the chromatin material of the nucleus,

the volume of the nucleus and the cytoplasm are considerably in-

creased, the nuclear membrane breaks down, and the chromosomes

assume bizarre shapes and various sizes. They may be paired, curved,

or straight; "V" and "C" and reversed "L" shapes, figure 8's, and

grouped as tetrads. This is known as the diakinesis stage. The chromo-

somes are then lined up on a spindle in anticipation of the first of

the two maturation divisions.

Spermatogenesis in the frog is seasonal and is completed within

the testes. The walls of the seminiferous tubules produce spermato-

gonia which go through mitotic divisions and then the series of

nuclear changes (described above) without mitosis. This results in

the appearance, toward the lumen of each tubule, of clusters of ma-

ture spermatozoa. By the time of hibernation (October) all the

spermatozoa that are to become available for the following spring

breeding season will have matured. At this time the testis will ex-

hibit only these spermatozoa and relatively few spermatogonia, with-

out the intervening maturation stages. The spermatogonia are found

close to the basement membrane of the seminiferous tubule. These

then await their turn to undergo the maturation changes necessary

for the production of spermatozoa which will be ready for the breed-

ing season a year and a half thereafter. The elongated and filamentous

April May June July August September October November Dec- Mor.

INTERSTITIALTISSUE

THUMB PAD z"

MATURATION

Normal cyclic changes in the primary and secondary sexual characters of the

frog, Rana pipiens. (From Glass and Rugh, 1944, J. MorphoL, 74:409.)


Meiotdivision

/«fw.*''^1«V'?

Spermatozoa -l*^**^.. « // /^^-^

Spermatogonia^^** "^^Jfj-*t •^•'' %

.

Sperm bundle Sertoli cell nucleus Interstitial tissue,

[^'spermatocytes\ Tunica olbuginea Spermatozoa Low cuboidal epithelium;

>*,

jLumen\ Sertoli cell nuclei \ Spermatids \ Seminiferous

Spermatids Spermatogonia ^'spermatocyte tubulesJunction

Collecting

tubule

Spermatogenesis in the frog: Rana pipiens. (A) Testis of a recently metamor-

phosed frog. (B) Similar to "A" except that the frog was previously treated with

the anterior pituitary hormone to evacuate the seminiferous tubules, leaving only

{Continued on facing page.)

THE MALE 37

Primary

spermatocyte

Secondaryspermatocytes

Sertoli cell

Spermatogonia

Second maturationdivision figures

Spermatids

Lumen of tubule

Heads of

mature spermatozoa

Spermatogenetic stages in the seminiterous tubule of the frog testis.

tails of the clustered mature spermatozoa project into the lumen of

each seminiferous tubule.

If one studies the July or August testis, which organ is then in

its height of spermatogenetic activity, he can find all the stages of

(Continued from opposite page.)

the interstitial tissue, spermatogonia, and a few primary spermatocytes. Thepost-breeding condition. (C) High-power magnification of "A." (D) High-power

magnification of "B." (E) Testis of an August frog, showing all stages of

spermatogenesis. (F) Partially (pituitary) activated testis, similar to "E," show-

ing released spermatozoa within the lumen, and all stages of spermatogenesis

around the periphery of the seminiferous tubule. (G) Testis of a hibernating,

pre-breeding frog. Note the clusters of mature spermatozoa attached to single

Sertoli cells. The lumen is filled with tails, few spermatogonia, and primary

spermatocytes around the periphery of the seminiferous tubule. (H) Testis of

the male during amplexus, showing spermatozoa liberated into the lumen of the

seminiferous tubule. (I) Collecting tubule of the frog testis full of mature sper-

matozoa, showing their origin from the connecting seminiferous tubules. Col-

lecting tubules are lined with low cuboidal epithelium and are continuous with

the vasa efferentia and the Malpighian corpuscles of the kidney.


maturation from the spermatogonium to the spermatozoon. Thespermatogonia are always located around the periphery of the semi-

niferous tubule and are small, closely packed cells, each with a

granular, oval nucleus. In between the spermatogonia may be found

occasional very large cells, the primary spermatocytes. These tend to

be irregularly spherical, possessing large and vesicular nuclei. Thecells are so large that they may be seen under low power magnifica-

tion (X 100) of the microscope. Apparently they divide to form

secondary spermatocytes almost immediately, for they are so few

and far between. The secondary spermatocytes (which develop as the

result of the first division) are about half the size of the primaries,

and lie toward the lumen of the tubule. They generally have a darkly

staining nucleus, and the cytoplasm may be tapered toward one

side. The spermatid, following another division, is even smaller and

possesses a condensed nucleus of irregular shape. Clusters of sperma-

tids appear as clusters of granules, the dark nucleus being almost as

small as the cross section of a sperm head. The metamorphic stages

from spermatid to spermatozoon are difficult to identify with ordinary

magnification, and are often confused with the spermatids themselves.

During this change the inner of two spermatid centrioles passes into

the nucleus while the outer one gives rise to the

tail-like flagellum.

The mature spermatozoon averages about 0.03

mm. in length. It has an elongated, solid-staining

head (nucleus) with an anterior acrosome, point-

ing outwardly toward the periphery of the semi-

niferous tubule. The short middle piece generally

is not visible but the tail appears as a gray fila-

mentous extension into the lumen, about four or

more times the length of the sperm head.

In any cross section of the testis, bundles of

sperm heads or tails may be cut at right angles

or tangentially, giving misleading suggestions of

structure. The mature spermatozoon is dependent

upon external sources of nutrition so that it joins

from 25 to 40 other spermatozoa, all of whose heads may be seen

converging into the cytoplasm of a relatively large, columnar-type

basal cell known as the Sertoli cell. This is functionally a nurse cell,

supplying nutriment to the clusters of mature spermatozoa until such

Frog spermato-

zoon. Total length

0.03 to 0.04 mm.

THE MALE 39

- Spermatozoa in

Malpighion

corpuscle

SpermotozoGpacked in unniferous

tubule

Bowman's capsule \ Malpighian

Glomerulus \ corpuscle

Spermatozoa

Glomerulus

Bowman's capsule

Vas efferens

full of sperm

Kidney of the male frog during amplexus showing spermatozoa in the kidney

tubules and Malpighian corpuscles, en route to the uro-(mesonephric) -genital

(vasa efferentia) duct.

time as they may be liberated through the genital tract to function in

fertilization.

In observing a section of the summer testis of the frog under low

power magnification, it is readily apparent that each seminiferous

tubule may contain all the stages of maturation and that each stage

is found in a cluster or group within the tubule. Each group of

similar cells is derived presumably from a single original spermato-

gonium, by the processes of mitosis and meiosis. This is reminiscent

of the condition found in the grasshopper (Rhomaleum) testis.

Maturation of the germ cells occurs in groups so that when the

spermatid stage is reached, the tips of the metamorphosing spermato-

zoon heads are all gathered together into the cytoplasm of the Sertoli

cell. Spermatozoa may remain thus throughout the entire period of

hibernation only to be liberated under the influence of sex-stimulating

hormones during the early spring. These spermatozoa are functionally


tectum inferior colliculus ^thalamus

nucleus N.m

nucleus N.iz

tegmentum --pg^s dorsalis

tiypothaJamus l

'pars ventrolis

WARNING CROAK

SPAWNING MOVEMENTS^ RELEASE

olfactory bulb

cerebral hemisphere

preoptic area

SWIMMING RESPONSE

Diagrammatic sagittal section through the brain of Rami pipiens indicating

the regions of the brain that were found to be of primary importance for the

mediation of each of four phases of sexual behavior. (From Aronson, 1945,

Bull. Am. Mas. Nat. Hist., 86:89.)

mature, as can be demonstrated by dissecting them from the testes

and using them to fertihze frogs' eggs artificially at any time from

late in August until the normal breeding season in April or May.

Reproductive Behavior

It has been proved definitely that the anterior pituitary hormone

causes the release of the mature spermatozoa from the testis. But

this hormone also releases other maturation stages. It is therefore

probable that there are smooth muscle fibers, either among the in-

terstitial cells or in the tunica albuginea of the testes, which fibers

contract to force the spermatozoa from the seminiferous tubules.

It would be as difficult to physiologically demonstrate the presence of

these fibers in the testis as it is simple to demonstrate them in the

contracting cyst wall of the ovary.

Responding to sex stimulation, the spermatozoa become free from

their Sertoli cells and are forced from the lumen of the seminiferous

tubule into the related collecting tubule. These collecting tubules are

small and are lined with closely packed cuboidal cells. They join

the vasa efferentia which leave the testis to pass between the folds

of the mesorchium and thence into the Malpighian corpuscles of the

kidney. From this point the spermatozoa pass by way of the excre-

tory ducts, the uriniferous tubules, and into the mesonephric duct

THE MALE 41

(ureter) which may be found attached to the lateral margin of the

kidney. Within the excretory system the spermatozoa are immotile,

due to the slightly acid environment. They are carried passively down

the ureter to the slight dilation near the cloaca, known as the seminal

vesicle. Within the vesicle the spermatozoa are stored briefly in clus-

ters until amplexus and oviposition occur. At oviposition the male

ejaculates the spermatozoa into the neutral or slightly alkaline water

where they are activated and then are able to fertilize the eggs as they

emerge from the cloaca of the female.

During the normal breeding season amplexus is achieved as the

females reach the ponds where the males are emitting their sex calls.

During amplexus there are definite muscular ejaculatory movements

on the part of the male frog, coinciding with oviposition on the

part of the female. Amplexus may be maintained by the male for

many days, even with dead females. As soon as the eggs are laid and

the male has shed his sperm, he goes through a brief weaving mo-

tion of the body and then releases his grip to swim away. The frogs

completely neglect the newly laid eggs.

Accessory Structures

In the male frog the ureter is not directly connected with the

bladder, as it is in higher vertebrates. It is possible that the bladder in

the Anura may be an accessory respiratory and hydrating organ,

particularly in the toads, where water may be stored during migra-

tions onto land.

The male frog also has a duct, homologous to the oviduct of the

female, known as the "rudimentary oviduct" or Miillerian duct. This

duct normally has no lumen, and is very much reduced in size so

that it may be difficult to locate. There is experimental evidence that

this duct may be truly a vestigial oviduct since it responds to ovarian

or female sex hormones by enlarging and acquiring a lumen.

At the anterior end of the testes of some Anura (e.g., toads) there

may be found an undeveloped ovary known as Bidder's organ. This

structure is said to respond to the removal of the adjacent testis or

to the injection of female sex hormones by enlarging to become struc-

turally like an ovary. Occasionally isolated ova have been found

within the seminiferous tubules of an otherwise normal testis, suggest-

ing the similar origin and the fundamental similarity of the testis and

the ovary.

Urogenital system of the male frog. The Miillerian ducts (vestigial oviducts)

arc seen lateral to the kidneys. They respond to female sex-stimulating hormones.

They converge with the Wolffian (mesonephric) duct at the cloaca. Note that

the left testis is usually smaller than the right. The vasa efferentia pass between

the folds of the mesorchium into the dorsally placed kidneys where they join the

uriniferous tubules at the glomeruli.

42

THE FEMALE 43

Finally, attached to the anterior end of the testis of the hibernating

frog may be seen finger-like fat bodies (corpora adiposa) which repre-

sent stored nutrition for the long period of hibernation, and for the

pre-breeding season when food is scarce. Under the microscope these

fat bodies appear as clusters of vacuolated cells, and are not to be

confused with the mesorchium. It is believed that they, as well as

the gonads, arise from the genital ridges of the early embryo. The fat

bodies tend to be reduced immediately after the breeding season, only

to be built up again as the time for hibernation approaches.

The Female

Secondary Sexual Characters

The mature female frog is generally larger than the male of the

same age and species, the Rana pipiens female measuring from 60 to

1 10 mm. in length from snout to anus. The sexually mature female

has a body length of at least 70 mm. It can be identified by the ab-

sence, at any season, of the dark thumb pad; the inability to produce

lateral cheek pouches resulting from the croaking reaction; a flabby

and distended abdomen; and the presence of peritoneal cilia. These

cilia are developed in the female in response to the prior development

and secretion of ovarian hormones.


The Ovaries.

The ovaries of the frog are paired, multi-lobed organs, attached

to the dorsal body wall by a double-layered extension of the peri-

toneum known as the mesovarium. This peritoneum continues around

the entire ovary as the theca externa. Each lobe of the ovary is hollow

and its cavity is continuous with the other 7 to 12 lobes. The ovaries

of the female are found in the same relative position as the testes

of the male but the peritoneum extends from the dorso-mesial wall

rather than from the kidneys, as in the male.

The size of the ovary varies with the seasons more than does the

size of the testis. From late summer until the spring breeding season

the paired ovaries will fill the body cavity and will often distend the

body wall. They may contain from 2,000 {Rana pipiens) to as manyas 20,000 eggs {Rana catesbiana) , each measuring about 1.75 mm.in diameter {Rana pipiens). The mature eggs are highly pigmented


on the surface of the animal pole, so that the ovary has a speckled

appearance of black pigment and white yolk, representing the animal

and the vegetal hemispheres of the eggs.

There is no appreciable change in the size of the ovary during

hibernation, nor is there any observable cytological change in the

ova. However, if a female is forced to retain her eggs beyond the

normal breeding period by isolating her from males or by keeping her

in a warm environment and without food, the ova will begin to de-

teriorate (cytolize) within the ovary. Immediately after the spring

breeding season, when the female discharges thousands of mature ova,

the remaining ovary with its oogonia (to be developed for the fol-

lowing year) is so small that it is sometimes difficult to locate. There

is no pigment in the tissue of the ovary (in the stroma or in the im-

mature ova), and each growing oocyte appears as a small white sphere

of protoplasm contained within its individual follicle sac.

Late development of the frog ovary.

THE FEMALE 45

The histology of the ovary shows that within its outer peritoneal

covering, the theca externa, are suspended thousands of individual

sacs, each made up of another membrane, the theca interna or cyst

wall, which contains smooth muscle fibers. This theca interna is

derived from the retro-peritoneal tissue. The smooth muscle fibers

can be seen histologically and can be demonstrated physiologically.

The theca interna surrounds each egg except for the limited area

bulging toward the body cavity, where it is covered by only the theca

externa. This is the region which will be ruptured during ovulation

to allow the egg to escape its follicle into the body cavity. The theca

interna, plus the limited covering of the theca externa, and the follicle

cells together comprise the ovarian follicle. These two membranes

make up the rather limited ovarian stroma of the frog ovary, and

they contain both blood vessels and nerves. Within each follicle are

found follicle cells, with their oval and granular nuclei, derived origi-

nally from oogonia. These follicle cells surround the developing

oocyte and are found in close association with it throughout those

processes of maturation which occur within the follicle. Enclosed

within the follicle cells, and closely applied to each mature egg, is

the non-cellular and transparent vitelline membrane, probably de-

rived from both the ovum and the follicle cells. This membrane is

developed and applied to the egg during the maturation process so

that it is not seen around the earlier or younger oogonia. Since the

bulk of the egg is yolk (vitellus), this membrane is appropriately called

the vitelline membrane. It is sometimes designated as the primary

(of several) egg membranes. After the egg is fertilized this mem-brane becomes separated from the egg and the space between is then

known as the perivitelline space, filled with a fluid. The fluid maybe derived from the egg which would show compensatory shrinkage.

As the oocyte matures and enlarges, the follicle cells and mem-branes are so stretched and flattened that they are not easily distin-

guished. It is therefore best to study these structures in the immature

ovary.

The egg will mature in any of a variety of positions within its

follicle, the exact position probably depending upon the maximumblood supply. As one examines an ovary the eggs will be seen in all

possible positions, some with the animal hemisphere and others with

the vegetal hemisphere toward the theca externa and body cavity. It

is believed that the most vascular side of the follicle wall will tend


i/.m.

Small ovarian egg of the frog surrounded by its follicle (/.) and theca {th.),

which is continued into the pedicle (p.). {h.v.) A blood vessel between follicle

and theca. (v.m.) Vitelline membrane, (c/z.) Chromatin filaments, now achro-

matic. («.) Chromatic nucleoli, ejected from the nucleus (n'.) and becoming

achromatic (/!".). (Courtesy, Jenkinson: "Vertebrate Embryology," Oxford, The

Clarendon Press.)

Section of a mature ovarian egg to show the area of ultimate follicular rupture

and the surrounding membranes of the egg.

THE FEMALE 47

Blood vessel

Theca externa

ThecQ interna

Theca externa

ThecG interna

Follicle cells

Figure 2

Theca interna

Pigment

Figure 3

Point of follicular rupture

Follicle ceils | ^^/ Theca externa

Blood vessel

Nucleolus

Point of follicular rupture

1

Follicle cell

Follicle cells

Theca interna

Theca externa

Blood vessel

Figure 4

Vegetal pole

GROWING OOCYTES OF THE FROG


to produce the animal hemisphere of the egg, and hence give it its

fundamental symmetry and polarity.

The frog's egg is essentially a large sac of yolk, the heavier and

larger granules of which are concentrated at the vegetal pole. There

is a thin outer layer of cytoplasm, more concentrated toward the ani-

mal hemisphere and in the vicinity of the germinal vesicle or imma-

ture nucleus. Surrounding the entire egg is a non-living surface coat,

also containing pigment. This pigment is presumably a metabolic by-

product. This coat is necessary for retaining the shape of the egg and

in aiding in the morphogenetic processes of cleavage and gastrulation

(Holtfreter).

The Body Cavity and the Oviducts.

Lateral to each ovary is a much-coiled oviduct suspended from

the dorsal body wall by a double fold of peritoneum. Its anterior end

Reactions of the frog ovary to stimulation by the frog anterior pituitary hor-

mone. (A) Ovary of a female receiving inadequate injection of the pituitary

hormone, partially emptying the eggs into the body cavity. (B) Ovary of a female

receiving almost enough pituitary hormone to empty all of its follicles, one lobe

alone retaining some of its eggs. During the breeding season the female's own

pituitary gland is sufficient to bring about complete ovulation of all eggs.

THE FEMALE 49

Photograph of Rana pipiens female body cavity at the height of ovulation.

is found between the heart and the lateral peritoneum, at the apex

of the liver lobe. At this anterior end is a slit-like infundibulum or

ostium tuba with ciliated and highly elastic walls. The body cavity

of the female is almost entirely lined with cilia, each cilium having

its effective beat or stroke in the general direction of one of the ostia.

These cilia are produced in response to an ovarian hormone and

therefore are regarded as secondary sex characters. They are found

on the peritoneum covering the entire body cavity, on the liver, and

on the pericardial membrane. There are no cilia on the lungs, the

intestines, or the surface of the kidneys except in the ciliated peri-

stomial (peritoneal) funnels which lead into the blood sinuses of the

kidneys. The abundant supply of cilia of the female means that eggs

ovulated from any surface of the ovary will be carried by constant


ciliary currents anteriorly toward and into one or another of the

ostia. This can be demonstrated easily by opening the body cavity

of an actively ovulating frog or by excising a strip of ventral abdom-

inal wall of the adult female, inverting it in amphibian Ringer's solu-

tion, and placing on it some of the body cavity eggs. Any object of

Deposition of jelly on the frog's egg. (Top) String of eggs removed from the

oviduct to show the deposition of jelly. The jelly is swollen in water. (Bottom)

Frog's eggs showing varying amounts of jelly, indicating progressive deposition

along the oviduct. (A) From upper third of oviduct. (B) From middle of

oviduct. (C) From body cavity (no jelly). (D) From uterus.

Immature oocytes Mature ovum emerging from, follicle

Portion of egg Emerged portion Point of follicle Theca Empty (collapsed) Immaturestill in ovarian of egg rupture externa follicle sacs oocytes

of ovary

Follicle wall

\Emerging egg

^ Two -thirdsemerged

Half emerged

Fully emergednote shape)

3 ^^ '

__ ^^p emerged ^^m^^^

^^^^ ^^m H^^^l^lî, ^ (note

Follicular rupture and ovulation in the frog. {Top, left) Three eggs emerging

simultaneously from their follicles. {Top, right) Eggs in various stages of emer-

{Continiicd on facing page.)

52

THE FEMALE 53

Photograph of the open body cavity of an actively ovulating female frog showing

the entrance of an egg into the ostium.

literally squeezed from the follicle, through a small aperture.

The process looks like an Amoeba crawling through an inadequate

hole. Ovulation (rupture and emergence of the egg) takes several

minutes at laboratory temperatures, and is not accompanied by

hemorrhage. By the time the egg reaches the ostium (within 2 hours),

as the result of ciliary propulsion, it is again spherical.

Ciliary currents alone force the egg into the ostium and oviduct.

The ostial opening is very elastic and does not respond to the respira-


gence and adjacerrt empty follicles. (Center) Egg about to drop free into the

body cavity, showing degree of constriction by the follicular opening. (Bottom)

Excised follicles of an ovulating frog continuing the process of emergence of

eggs. Note the plasticity of the egg at this time.


tory or heart activity, as some have described. The eggs are simply

forced into the ostium, from all angles, stretching its mouth open to

accept the egg. As soon as the egg enters the oviduct and begins to

acquire an albuminous (mucin-jelly) covering, it becomes fertiliza-

ble. One can remove such an egg from the oviduct by pipette or by

cutting the oviduct 1 inch or more from the ostium, and can fertilize

such an egg in a normal sperm suspension. The physical (or chemical)

changes which occur between the time the egg is in the body cavity

and the time it is removed from the oviduct, which make it fertiliza-

ble, are not yet understood.

As the egg is propelled through the oviduct by ciliary currents,

it receives coatings of albumen (jelly). The initial coat is thin but

of heavy consistency, and is applied closely to the egg. The egg is

spiraled down the oviduct by its ciliated lining so that the application

of the jelly covering is quite uniform. There are, in all, three distinct

layers of jelly, the outermost one being much the greater in thick-

ness but the less viscous. The intermediate layer is of a thin and more

fluid consistency. There is hyperactivity of the glandular elements of

the oviduct just before the normal breeding season, or after anterior

pituitary hormone stimulation, so that the duct is enlarged several

times over that of the oviduct of the hibernating female.

The presence of the jelly layers on the oviducal or the uterine egg

Distribution of coelomic cilia within the body cavity of the female frog. (Left)

Schematic section through the level of the ovaries. (Right) Schematic drawing

of the open body cavity. The cilia in the body cavity of the female develop in

response to the elaboration of an ovarian hormone, and function in propelling

the eggs to the two ostia.

THE FEMALE 55

is not readily apparent because it requires water before it reaches

its maximum thickness. Eggs sectioned within the oviduct show the

jelly as a transparent coating just outside the vitelline membrane. Assoon as the egg reaches the water, however, imbibition swells the

jelly until its thickness becomes greater than the diameter of the

egg-

The function of the jelly is to protect the egg against injury, against

ingestion by larger organisms, and from fungus and other infections.

Equally important, however, is the evidence that this jelly helps the

egg to retain its metabolically derived heat so that the jelly can be

said to act as an insulator against heat loss. Bernard and Batuschek

(1891) showed that the greater the wave length of light the less

heat passed through the jelly around the frog's egg, in comparison

with an equivalent amount of water and under similar conditions.

Oviducal blood vessels~*%

Passage of eggs through the oviduct. The eggs of the frog are greatly distorted

as they pass down the oviduct toward the uterus. They accumulate albumenaround them, but, since they spiral down the duct, the albumen jelly is evenly

deposited and the eggs become spherical as the jelly swells when the eggs pass

from the uterus into the water.


Oviducts of the frog under various states of sexual activity. (A) Post-ovula-

tion condition, collapsed and dehydrated. (B) Actively ovulating condition,

oviduct full of eggs, edematous. (C) Oviduct of non-ovulating, hibernating

female.

Originally, and erroneously, the jelly was thought to act as a lens

which would concentrate the heat rays of the sun onto the egg, but

since the jelly is largely water, which is a non-conductor of heat

rays, this theory is untenable. One can demonstrate that the tempera-

ture of the egg is higher than the temperature of the immediate en-

vironment, even in a totally darkened environment. So, the jelly has

certain physical functions in addition to those as yet undetermined

functions which aid in rendering the egg fertilizable.

The egg takes about 2 to 4 hours, at ordinary temperatures, to

reach the highly elastic uterus, at the posterior end of the oviduct

and adjacent to the cloaca. Each uterus has a separate opening into

the cloaca, and the ovulated eggs are retained within this sac until,

during amplexus (sexual embrace by the male), they are expelled into

the water and are fertilized by the male. Generally the eggs are not

retained within the uterus for more than a day or so. There may be

quite a few hours between the time of appearance of the first and

the last eggs in the uteri.

THE FEMALE 57

Oogenesis—Maturation of the Egg.

The maturation process can best be described as it begins, imme-

diately after the normal breeding season in the spring. At this time the

ovary has been freed of its several thousand mature eggs and con-

tains only oogonia with no pigment and little, if any, yolk. Even

at this early stage each cluster of oogonia represents a future ovarian

unit, consisting of many follicle cells and one ovum. There has been

no way to determine which oogonium is to be selected for maturation

into an ovum and which will give rise to the numerous follicle cells

that act as nurse cells for the growing ovum. It is clear, however, that

both follicle cells and the ovum come from original oogonia. All ova

develop from oogonia which divide repeatedly. These pre-maturation

germ cells divide by mitosis many times and then come to rest, dur-

ing which process there is growth of some of them without nuclear

division. These become ova while those that fail to grow becomefollicle cells. However, there are pre-prophase changes of the nucleus

of the prospective ovum comparable to the pre-prophase changes in

spermatogenesis. The majority of oogonia, therefore, never mature

into ova, but become follicle cells.

11'i V'

9

Prophases of the heterotypic division in the female (ovary of tadpole). (1)

Nucleus of oogonium. (2) Leptotene. (3) Synaptene. (4, 5) Contraction figures.

(6) Pachytene. (7) Later pachytene, multiplication of nucleoli. (8, 9) Diplo-

tene: the chromatin filaments are becoming achromatic; granules of chromatinare being deposited on the nucleoli. (Courtesy, Jenkinson: "Vertebrate Em-bryology," Oxford, The Clarendon Press.)


Normal nuclear growth cycle of the ovum of Rana pipiens. (Stage 1 ) Smallest

follicle in which the chromosomes within the germinal vesicle can be seen.

(Stage 2) The paired chromosomes are barely visible, embedded in a nucleo-

plasmic gel. Egg diameters less than 200 microns. (Stage 3) Eggs measuring from

200 to 500 microns in diameter, more detail visible through the transparent theca

cells. Lateral loop production begins. Zone of large irregular nucleoli may be

seen just beneath the nuclear membrane. (Stage 4) First development of yellow-

brown color and yolk. Eggs range in size from 500 to 700 microns in diameter.

Chromosomes attain length of about 450 microns. For salamanders of com-

parable stage chromosomes measure 700 microns in length. (Stage 5) Chromo-

some frame begins contraction while the nucleus continues to grow in eggs

ranging in diameter from 750 to 850 microns. This is approximately half the

ultimate size. Chromosomes shorten and have fewer and smaller loops. Themajor nucleolar production continues and sacs appear on the surface of the

nuclear membrane. (Stage 6) Egg diameter about 1.8 millimeters and germinal

vesicle is of maximum size. Chromosome frame now about 1/1000 of the nuclear

volume, coated by a denser substance which can be coagulated by the calcium

ion. Chromosomes have shortened to 40 microns or less and have lost all large

and small hyaline bodies called loop fragments.

Heavy arrows indicate the mixing of nuclear material in the cytoplasm after

the breakdown of the germinal vesicle. The dotted arrow indicates migration of

the central chromosomal mass toward the animal pole to become the maturation

spindle for the first polar body. (From W. R. Duryee, 1950, Antu N, Y, Acad.

Sci., 50, Art. 8.)

THE FEMALE 59

The process of maturation involves contributions from the nucleus

and the cytoplasm. First, chromatin nucleoli aid in the synthesis of

yolk, and second, the breakdown of the germinal vesicle allows an

intermingling of the nuclear and the cytoplasmic components. Only

a small portion of the germinal vesicle is involved in the maturation

Ovarial wall of Rami temporaria. Note young trans-

parent eggs (stages 1 and 2) and larger opaque eggs

{stage 3). The arrow points to an isolated nucleus

from another egg (stage 3) , which has floated into the

field. (Courtesy, W. R. Duryee, 1950, Ann. N. Y.

Acad. ScL, 50, Art. 8.)

spindle so that it may be at this time that the nucleus exerts its initial

influence on the cytoplasm. All cytoplasmic differentiations must be

initiated at a time when the hereditary influences of the nucleus are

so intermingled with it.

Growth Period to Primary Oocyte Stage. Growth is achieved

largely by the accumulation of yolk. As soon as growth begins the

cell no longer divides by mitosis and is known as an oocyte rather

than an oogonium. The growth process is aided by the centrosome,


which is found to one side of the nucleus, and around which gather

the granules or yolk platelets. The chromatin filaments become achro-

matic and the nucleoli increase in number, by fragmentation, and

become more chromatic. Many of the nucleoli, which are concen-

trations of nucleo-protein, pass through the nuclear membrane into

the surrounding cytoplasm during this period. It is not clear whether

this occurs through further fragmentation of the nucleoli into particles

of microscopic or sub-microscopic size, and then their ejection

through the nuclear membrane. It may occur by the loss of identity

(and chromatic properties) by possible chemical change and sub-

sequent diffusion of the liquid form through the membrane to be

1= J100-^

The entire set of chromosomes of Rana lemporaria. The 1 3 pairs of chromo-

somes in this species are remarkably like those of other species of Rana. They

are seen in stage 6. (Q) Four ring-shaped pairs. (R) Four medium-sized pairs.

(S) Three longest (super) pairs. (T) Two short T-shaped pairs. (Courtesy,

W. R. Duryee, 1950, Ann. N. Y. Acad. ScL, 50, Art. 8.)

THE FEMALE 61

FROG:

Similarity of frog and of salamander chromosome structure. Stage 4 chromo-

somes. There is apparently a single chromonema along which compound gran-

ules and chromomeres of varying shapes and sizes are firmly embedded or at-

tached. These chromosomes were treated with 0.2 M NaHCOa to remove the

lateral loops and reveal the chromonemata. Chromosome granules are attached

to the paired chromonemata at homologous loci. Some matric coating is present.

Mild acidification following carbonate treatment has removed most of the lateral

loops. (Courtesy, W. R. Duryee, 1950, Ann. N. Y. Acad. ScL, 50, Art. 8.)

resynthesized on the cytoplasmic side of the membrane. During the

growth of the oocyte, further nucleoli appear within the nucleus,

only to fragment and later to pass out into the cytoplasm. The

presence of chromatic nucleoli in the cytoplasm is closely associated

with the accumulation (deposition) of yolk.

The granules within the cytoplasm (extruded fragments of nucleoli)

function as centers of yolk accumulation and have therefore been

named "yolk nuclei." This is an unfortunate name, for the structure

is a nucleus only in the sense that it is a center of aggregation. It is

not a true cell nucleus. The centrosome and other granular centers

lose their identity and the yolk granules then become scattered

throughout the cytoplasm.

The source of all yolk for the growing ova is originally the digested

food of the female. This nutrition is carried to the ovary by way of

the blood system and conveyed to the nurse or follicle cells and thence

to the oocyte. The yolk is at first aggregated around yolk nuclei, then

concentrated to one side of the nucleus. Finally it assumes a ring

shape around the nucleus between an inner and an outer zone of

cytoplasm. Subsequently the nucleus is pushed to one side by the

ever-increasing mass of yolk so that eventually there is an axial


Lateral loops of the amphibian chromosome. The lateral loops originate from

chromosomal granules and the lateral branches are not homogeneous in struc-

ture, but are made up of smaller particles embedded in a hyaline cylinder. These

lateral loops occur in separable clusters of 1 to 9 loops along a single chro-

monema. These loops reach their greatest development at stage 4, when the

chromosome frame is most expanded. They average 9.5 microns in length but

may reach 24 microns. They are not resorbed back into the chromosome and

the number of loops per chromosome decreases with time, although the number

of chromomeres per chromosome remains constant. (Courtesy, W. R. Duryee,

1950, Ann. N. Y. Acad. ScL, 50, Art. 8.)

gradient of concentration of oval yolk platelets from one side of the

egg to the other. The smaller platelets are found in the vicinity of

the nucleus, in the animal hemisphere. The larger platelets are lo-

cated toward the vegetal hemisphere. There is an increase averaging

from 200 to 700 per cent in the total lipoid substance, neutral fat,

total fatty acids, total cholesterol, ester cholesterol, free cholesterol.

THE FEMALE 63

^..r^.'-^:

Paired chromosomes showing

^'chromomeres and side loops ^^'"^, '*'"":'^*'-''^^

» '

Pair of chromosomes

&i^. i ;; exchanging secticms


and phospholipin content of the ovaries of Rana pipiens occurring

during the production and growth of ova (Boyd, 1938). The primary

oocyte may show a sHght flattening of the surface directly above

the region of the nucleus.

These growth changes and the unequal distribution of pigment,

yolk, and cytoplasm are the first indications of polarity or a gradient

system within the egg. When the polarity is well established, the

cytoplasm, the superficial melanin or black pigment, and the nucleus

are all at the animal hemisphere (pole). The light colored yolk is

more concentrated toward the vegetal pole. The egg is then re-

garded as a telolecithal egg. During this phase of egg maturation

there is a drain on the metabolism of the frog which requires an ex-

cess of food intake because the materials for egg growth must be

synthesized from nutritional elements received from the vascular

system of the female. For Rana pipiens this period of most active

feeding comes during the summer when the natural foods, insects,

worms, etc., are the most abundant.

During the growth of the oocyte in general there are important

changes occurring within the nucleus (germinal vesicle) of the egg.

Thirteen pairs of chromosomes may be

seen in synizesis (contraction), converg-

ing toward the centrosome at the "yolk

nucleus" stage. A little later the nuclear

membrane develops sac-like bulges, the

nucleoli are scattered, and there is a

colloidal chromosome core which almost

fills the entire nucleus. The chromo-

somes themselves are small and almost

invisible. When the primary oocyte is

about half its ultimate size, there appear

definite sacs on the nuclear surface. The

fragmented nucleoli are located at the

periphery of the lobulated nuclear mem-brane, and the chromosome frames have

become relatively large. The chromo-

somes, by this time, have reached their

maximum length and possess large lateral loops. Finally, in the fully

grown nucleus of the primary oocyte the nuclear sacs are very

prominent, and the nucleoli appear in clusters in the center of the

Diploid metaphase chromo-

somes from the tail fin of a 15-

day-old Rana pipiens tadpole.

(Courtesy, K. R. Porter, 1939,

Biol. Bull., 77:233.)

THE FEMALE 65

egg, surrounding the chromosome frame. This frame is a gel struc-

ture which gives rise to the first maturation spindle, containing 13

pairs of slightly contracted chromosomes.

These structural features can be observed in the living germinal

vesicle if it is removed from the oocyte and placed in isotonic and

balanced salt medium, omitting the calcium ion. A minute amount of

NaHoPO, is added to shift the pH toward the acid side, which makes

the chromosomes the more visible. Or, the chromosomes may be

Colloid ground

substance

Central

chromosome core

Nuclearmembrane

Isolated germinal vesicle (nucleus) of Rami pipiens. Nucleus isolated in

calcium-free Ringer's solution, from stage 6, showing sac-like organelles which

protrude from the nuclear membrane. The cloud of central nucleoli surrounds

and obscures the chromosome frame. (Courtesy, W. R. Duryee, 1950, Ann.N. Y. Acad. Sci., 50, Art. 8.)


i

THE FEMALE 67

accumulations of spherical, black pigment granules. With each cleav-

age, subsequent to fertilization, this superficial coat is divided between

the blastomeres, being an integral part of the living cell. There is no

clear-cut demarcation between this surface coat and the inner cyto-

plasm and yolk. It is believed that these growth changes of the egg

are under the influence of the basophilic cells of the anterior pitui-

tary gland, which cells are greater in number at this time than at

any other.

During the growth period the vitelline membrane appears on the

surface of the oocyte as a thin, transparent, non-living, and closely

adherent membrane. It is formed presumably by a secretion from the

egg itself, aided by the surrounding follicle cells. It appears to be

similar in all respects to the membrane of the same name found

around the eggs of all vertebrates.

Ovulation and Maturation. Ovulation, or the liberation of the

egg from the ovary, is brought about by a sex-stimulating hormone

from the anterior pituitary gland. Just before and during the normal

spring breeding period there is a temporary increase in the relative

number of acidophilic cells in the anterior pituitary. It is believed that

this is not coincidental but a causal factor in sex behavior in the frog.

However, until such time as extracts of specific cell types can be

made, this will be difficult to prove conclusively. Attempts on the

part of the male to achieve amplexus are resisted by the female not

sexually stimulated. However, such a female can be made to accept

the male by injecting the female with whole anterior pituitary glands

from other frogs. It is very probable that environmental factors such

as light, temperature, and food may act through the endocrine sys-

tem to prepare the frogs for breeding when they reach the swampy

marshes in the early spring, after protracted hibernation. It must

be pointed out, however, that there are frogs in essentially the same

environments which breed in July (Rana clamitans) and August

{Rana catesbiana) , so that the causal factors appear to be either com-

plex or possibly different for different species. The pituitaries of the

hibernating frogs do contain the sex-stimulating factor, but apparently

to a lesser degree than the glands of frogs approaching the breeding

season. The injection of 6 glands from adult female frogs will cause

an adult female of Rana pipiens to ovulate as early as the last week

in August, some 8 months before the normal breeding period. Oneor two such glands will accomplish the same results if used early in


Production of ova and the process of ovulation. (Top) Section through a

primary o5cyte of Rana pipiens showing the absence of the vitelUne membrane,


THE FEMALE 69

The maturation divisions in the female (Axolotl). (1) First polar spindle with

heterotypic chromosomes. (2) Extrusion of first polar body. (3) Appearance of

second polar spindle. Longitudinal division of chromosomes in egg and in first

polar body. (4) Second polar spindle radial. Homoeotypic chromosomes on

equator (metaphase). (5) Polar view of the same. (6) Anaphase. (7) Extrusion

of second polar body. (8) Second polar body with resting nucleus. (9) Female

pronucleus in resting condition, closely surrounded by yolk granules. (Courtesy,

Jenkinson: "Vertebrate Embryology," Oxford, The Clarendon Press.)

[Continued from opposite page.)

the presence of the follicle and cyst wall, and the predetermined area of ultimate

follicular rupture. {Center) Mature ovum with axial gradient of pigment and

yolk. The vitelline membrane is now present, and the cyst wall of smooth muscle

cells is stretched. (Bottom) Ovarian egg partially emerged from its follicle dur-

ing ovulation. Note degree of constriction, contraction of cyst wall, and fully

developed vitelline membrane around the entire egg.


Endolymphatic

tissue

rior pituitary

ars nervosa

Optic chiasmaBrain

The position of the master endocrine gland—the anterior pituitary of Ranapipiens.

April. Another explanation for this may be offered, namely that the

ovary itself may become more sensitive to such stimulation as the

breeding season approaches.

The ovulation process itself consists of the rupture and the emer-

gence of eggs from their individual follicles. The surface of the egg,

separated from the body cavity by only the non-vascular theca externa,

is first ruptured and then the egg slowly emerges through the small

opening. Since the egg is known to contain a peptic-like enzyme, it is

believed that the pituitary hormone may activate this enzyme to

digest away the tight and non-vascular covering. Then by stimulation

of the smooth muscle fibers of the cyst wall (theca interna) the process

of emergence is completed. The relation of the pituitary to smooth

muscle activity has long been established clinically.

It is true that the ovarian stroma shows undulating contractions at

all seasons, irrespective of sex activity. An egg will emerge at any

time from a surgically ruptured follicle. If an ovulating female is

etherized and the body cavity is opened, the ovary may be removed

THE FEMALE 71

and placed in amphibian Ringer's solution and the ovulation process

may be observed directly. This will go on for several hours after all

connections with the nerve and blood supply are cut off. From the ini-

tial rupture of the theca externa until the egg drops free into the

body cavity there is a lapse of from 4 to 10 minutes at ordinary

laboratory temperatures.

The first maturation division occurs at the time of ovulation. The

heterotypic chromosomes (i.e., of bizarre shapes) are placed on the

spindle of the amphiaster whose axis is at right angles to the egg sur-

face. Movement of the chromosomes is identical with that found in

ordinary mitosis. The outermost group of telophase chromosomes are

pinched off, with a small amount of cytoplasm and no yolk, to com-

prise the first polar body. The innermost telophasic group of chromo-

somes remain within a clear (i.e., yolk-free) area of the egg as the

nuclear mass of the secondary oocyte. These changes occur as the egg

leaves the ovary and before it reaches the oviduct. Possibly the same

forces which bring about follicular rupture also influence this mat-

uration process.

The second maturation division begins without any intermediate

rest period for the chromosomes, at about the time the egg enters the

oviduct. There may be a variation in time up to 2 hours for eggs to

reach the ostium, depending upon the region of the body cavity into

which they are liberated. Thus the stage of maturation of different

eggs within the oviduct may vary considerably. There is a longitudi-

nal division of the chromosomes of the egg which are lined up in

metaphase on the second maturation spindle, the axis of which is at

right angles to the egg surface. Since the spindle is primarily proto-

plasmic, and is made up in part of fibers (which may be contractile),

the space occupied by the spindle will be free of yolk. Since it is

peripherally placed, and represents a slight inner movement after

the elimination of the first polar body, the surface layer of the egg is

slightly de-pigmented just above the spindle region. This situation is

exaggerated in aged eggs, a relatively large de-pigmented area of

the cortex appearing toward the center of the animal hemisphere.

Maturation is not completed until or unless the egg is activated bysperm or stimulated by parthenogenetic means. However, every egg

reaching the uterus is in metaphase of the second maturation division,

awaiting the stimulus of activation to complete the elimination of the

second polar body.

CHAPTER FOUR

Fertiliiâtion or tlie Prod S Ej

Completion of Maturation of the Egg Symmetry of the Egg, Zygote, andPenetration and Copulation Paths of Future Embryo

the Spermatozoon

"The unfertilized egg dies in a comparatively short time, while

the act of fertilization saves the life of the egg and allows it to give

rise, theoretically at least, to an unlimited series of generations. . . .

Fertilization makes the egg immortal." (J. Loeb, 1913.)

Fertilization generally is regarded as a two-fold process, involving

first the activation of the egg. This can be accomplished by the sperm

or by a variety of parthenogenetic agents. Second, there is the mixing

of hereditary (nuclear and chromosomal) potentialities, a process

known as amphimixis. Insemination means simply the exposure of

the eggs to spermatozoa. The most accepted evidence for activation is

an elevation of the vitelline membrane away from the egg and its

transformation into what is then called a fertilization membrane.

The vitelline membrane can be lifted from the ovarian eggs of the

frog in distilled water or calcium-free Ringer's solution at pH 7.5

to 10.2. This purely physical change seems to be the result of pro-

gressive cytolysis or coagulation of the egg cortex, and the membraneitself has the consistency of thin cellophane. There are, of course,

physiological changes in the egg accompanying fertilization which

are so dynamic that even the microscopic surface pigment granules

become violently active. The ultimate proof of successful fertilization

is, of course, the diploid condition of the chromosomes of the

zygote.

As the frog's egg is shed into the water, the male (in amplectic

embrace) simultaneously and by active expulsion movements sheds

clouds of spermatozoa over the eggs. Fertilization in Anura is there-

fore external. The jelly, deposited on each egg as it passed through the

oviduct, swells almost immediately thereafter, partially protecting

the egg against multiple sperm entrance. The swollen jelly appears to

be arranged in three layers, being twice the thickness of the egg diam-

73

74 FERTILIZATION OF THE FROG S EGG

Amplexus in the toad: Bujo jowlcri. Pectoral grip.

eter. Polyspermy, or multiple sperm invasion of the egg, occurs

naturally in some telolecithal eggs such as those of the bird. It is

also common in urodele eggs, but not so with the Anura (e.g., the

frog). More effective than the jelly in limiting fertilization to one

sperm is the negative chemical and/or physical reaction of the egg

toward any extra spermatozoa after one of them has made contact

with the egg surface. Simultaneously with sperm activation there ap-

pears an immune reaction of the egg to the invasion of any accessory

spermatozoon, even of the same species. In any case, one and only

one spermatozoon nucleus normally fuses with the egg nucleus. Anyaccessory spermatozoa which are successful in invading the egg

attempt to divide independently of the egg nucleus and then degen-

erate. Extensive polyspermy, which may occur in aged eggs, inter-

feres drastically with the cleavage mechanism and the eggs reach an

early cytolysis and death.

The effective frog spermatozoon always enters the egg in the

animal hemisphere. This does not deny the possibility of sperm en-

trance in the metabolically more sluggish vegetal hemisphere. The

sperm nucleus which conjugates with the egg nucleus is one that

FERTILIZATION OF THE FROG'S EGG 75

^\M. l}'.'

Oviposition and fertilization in Rcina. (A) Egg-laying posture of Rana

clamitans, just prior to the onset of oviposition: dorsal view. (B) Upstroke of the

male and the appearance of the first batch of eggs. (C) Downstroke of the

male and the formation of the surface film: dorsal view. (D) Typical amplexus

in Rana clamitans: lateral view. (E) Pre-release motions of paired Rana pipiens.

(Courtesy, Dr. L. R. Aronson, American Museum of Natural History.)

76 FERTILIZATION OF THE FROG'S EGG

Polar bodies

Animal hemisphere

Gray crescent

Vegetal hemisphere

Jelly coats (3)

itelline membrane

The egg of the frog 35 minutes after fertUization. Note the second polar body,

the slight depression at the animal pole, and the recession of the cortical pig-

ment toward the sperm entrance point, forming the gray crescent. The jelly is

thicker than the diameter of the egg.

enters by way of the animal hemisphere. The factors which limit

effective sperm entrance to the animal hemisphere may be physical

and/or chemical. In any case, the reaction is rapid and the entire

egg becomes resistant to excess sperm entrance. In Amphioxus the

sperm entrance is characteristically in the vegetal pole, and in other

forms there may be no such polar restrictions.

Sperm contact and penetration of the egg has two rather immedi-

ate effects. First, it seems to allow the superficial jelly to swell to its

maximum, by imbibition. The jelly on the unfertilized egg will expand,

but to a lesser degree. The expansion of the jelly is at quite a uniform

rate and can be observed under low power magnification against a

dark background. Within 5 minutes there has been about 30 per cent

swelling, in 15 minutes about 75 per cent swelling, and thereafter the

imbibition is slower. Eventually the thickness of the three jelly layers is

several times the diameter of the egg. Second, another effect of sperm

penetration is that there is an almost immediate loss of water from the

egg so that a space appears between the egg surface and the envelop-

COMPLETION OF MATURATION OF THE EGG 77

ing vitelline membrane now known as the fertilization membrane. This

is known as the perivitelline space, filled with a lluid, within which

the egg is free to rotate. Since the mature egg has a definite yolk-

cytoplasmic gradient, it slowly rotates within the gravitational field

(inside the fertilization membrane) until the black pigmented animal

hemisphere is uppermost and the heavier yolk-laden vegetal hemi-

sphere is lowermost. When eggs are artificially inseminated with frog

sperm in the laboratory, this rotation will be complete within an hour

after insemination and the egg mass will present a uniform black ap-

pearance from above, provided there is adequate water coverage.

Completion of Maturation of the Egg

At the time of insemination the frog's egg nucleus is in the metaphase

of the second maturation division, and the egg is provided with a

vitelline membrane and a covering of unswollen jelly. The activation

of the egg by the spermatozoon brings on a resumption of the matura-

tion process so that the suspended mitosis is completed, and the second

polar body is given off into the perivitelline space.

Occasionally at the time of insemination eggs will show a minute

polar body pit or fovea, a surface marking indicating the point of

previous emergence of the first polar body. The first polar body can

be found generally as a translucent bead, floating freely within the

perivitelline space in the general vicinity of the animal hemisphere.

Within 7 to 10 minutes (at ordinary laboratory temperatures) a dis-

tinct pit will appear in the animal hemisphere, and this marks the

position of the activated amphiaster. Apparently sperm activation

causes the whole amphiaster to recede slightly into the egg, or such

slight movement might be the result of cortical changes following ac-

tivation, since the amphiaster is of quite different consistency from the

balance of the egg. In any case, there appears a small pit measuring

about 16 microns in diameter and often near the first polar body.

Within another 15 to 20 minutes (a total of 25 to 30 minutes from

insemination) the pit seems to disappear but only because there is seen

emerging from it the second, and also translucent, polar body. Within

about 1 minute this second body (nucleus) has emerged and its diam-

eter measures about 26 microns, indicating that it was squeezed

through an orifice somewhat smaller than its own diameter. There re-

mains no evidence of any pit and, even though the egg nucleus is just

below the surface, there is no superficial evidence of its presence. The

78 FERTILIZATION OF THE FROGS EGG

egg nucleus now recedes into the egg, re-forms, and moves slightly in

the direction of the approaching sperm nucleus. The path of the egg

nucleus is not marked by any pigment as is the path of the invading

spermatozoon.

Penetration and Copulation Paths of the Spermatozoon

The sperm head generally makes a direct perpendicular contact

with the egg cortex, through the unswoUen jelly and the vitelline mem-brane. Usually the entire spermatozoon enters the egg, taking several

Polar body emergence: Rana pipiens. Note tension lines in the cortical layer

caused by the 26-micron polar body emerging through a 16-micron opening.

PENETRATION AND COPULATION PATHS OF THE SPERMATOZOON 79

.^* Vii

il:-'#

5. 4:

The four stages in polar body emergence in Rana pipiens. (7) Division spindle

as in egg at time of insemination. (2) Anaphase of maturation division. Stage

at which spindle can be seen from exterior of the egg as a black dot. Egg fixed

35 minutes after insemination. (5) Early telophase. Egg fixed 50 minutes after

insemination. (4) Polar body just forming. Egg fixed 56 minutes after insemina-

tion. (Courtesy, K. R. Porter, 1939, Biol. Bull., 77:233.)

minutes for the complete invasion of the cortex. The tail piece may be

broken off, but the head and middle piece continue through the egg

substance in the general direction determined by the direction of pene-

tration, usually along the egg radius. This is therefore known as the

penetration path. Almost immediately, however, the sperm head en-

larges and loses its identity and is difficult to find by any known

cytological method.

In the process of invading the egg, the spermatozoon takes along

with it some of the pigment granules of the surface (cortical) layer so

that a cone-shaped penetration path can be seen in sections cut in a

parallel plane. This path is rather straight, indicating a certain amount

of directional force on the part of the penetrating spermatozoon. Thepigment path is the only evidence of the sperm movement, since the


sperm nucleus is so indistinct that it cannot be located in sectioned

eggs.

The middle piece of the spermatozoon, containing the centrosome,

enters the egg behind and attached to the sperm head or nucleus. As

Point of sperm invasion

Pigmented cortex of

animal hemisphere

Penetration path

Copulation path

Yolk

Pigmented cortex of

animal hemisphere

Jelly

Penetration path

Point of sperm entrance

Copulation path

Invasion of the frog's egg by a spermatozoon. ( Top) The first step in fertiUzation,

the second {bottom) being syngamy or the fusion of the sperm and egg nuclei.

this sperin head (nucleus) swells and loses its identity and establishes

the penetration path, the associated centrosome divides. The resulting

centrosomes establish an axis at right angles to the direction of sperm

progression. Frequently, however, the sperm penetration path will not

PENETRATION AND COPULATION PATHS OF THE SPERMATOZOON 81

Fertilization of the frog's egg. (A) Sperm penetration and copulation paths in

the same direction, forming a straight line, in which case the first cleavage bisects

the gray crescent. (B) Sperm copulation path veers away from the penetration

path, causing the first cleavage furrow to deviate from bisecting the gray cres-

cent. (C) Under compression, the egg protoplasm is re-oriented, and the cleav-

age spindle comes to lie in the plane of the longest protoplasmic axis, regardless

of sperm activity. (D) Profile view showing sUght movement of the egg nucleus

toward the sperm nucleus, after the egg nucleus has given off its second polar

body.

These diagrams are simplified by omitting the changes attending nuclear dis-

solution, prior to syngamy. so that the nuclear paths in syngamy can be empha-

sized. Only in "A" does the first cleavage coincide with the embryonic axis and

bisect the gray crescent. {Solid arrows) Penetration path, gray crescent, embry-

onic axis, (broken arrows) cleavage plane, which occurs at right angles to the

mitotic spindle. Egg nucleus is clear, sperm nucleus is solid.

82 FERTILIZATION OF THE FROG'S EGG

lead the sperm nucleus directly to the egg nucleus, or to the final

position of the egg nucleus (which itself moves away from the cortex)

.

In these cases the sperm path is diverted so that it will meet the egg

nucleus. This new direction is designated as the copulation path be-

cause it results in the copulation (or fusion) of the two nuclei. By the

time the two nuclei meet, the two sperm centrosomes are then well

separated and are ready to form the division spindle for the first

cleavage. It is probable that the chromosomes from the two sources do

not pair off before the first division, which occurs within 2Vi hours

after insemination at normal laboratory temperatures.

Symmetry of the Egg, Zygote, and Future Embryo

As has been emphasized, the egg polarity and the radial (rotatory)

symmetry about its axis are established while it is in the ovary during

the process of growth or yolk acquisition. One can draw an imaginary

line from the center of the animal pole and through the nucleus which

will pass through the geometrical center of the egg, and such a line will

pass through the center of the vegetal hemisphere. This line repre-

sents the primary axis around which there is radial symmetry. The

effective spermatozoon may enter the egg at any point within the

animal hemisphere. This may be close to the egg nucleus (toward the

center of the animal hemisphere), almost at the equator, or between

these extreme positions. It is probably very seldom that penetration

occurs just above the second maturation spindle, in the center of the

animal hemisphere (i.e., in the egg axis). For this reason, it is safe to

assume that the majority of penetration points will be at some other

region of the animal hemisphere. This means that a third point is estab-

lished (i.e., by sperm penetration), the other two being any two

points on the linear axis of the egg. These three points will establish a

plane, the penetration path plane, which is of major importance in

establishing embryonic planes. The egg therefore loses its rotatory

symmetry at the moment of sperm penetration.

The significance of the sperm entrance point is brought out when it

is realized that it immediately establishes the antero-posterior axis or

bilateral symmetry of the future embryo. That side of the animal

hemisphere which is toward the sperm entrance will be toward the

anterior, the opposite will be toward the posterior, of the future

embryo. This antero-posterior plane (formerly the sperm penetration

path plane) separates the future embryo into right and left halves.

SYMMETRY OF THE EGG, ZYGOTE, AND FUTURE EMBRYO 83

Vitelline Polar bodiesibro

Surface changes of the frog's egg at the time of fertilization. (A) Sperm

contact, entrance generally in the animal hemisphere. (B) Second polar body

emerges, the sperm penetrates the cortex, and the surface pigment recedes

toward the sperm entrance point forming the gray crescent. Lateral view.

(C) Dorsal view showing the sperm penetration path, centrally located polar

bodies and egg nucleus, and (arrows) pigment recession to form the gray cres-

cent. (D) Rotation of the sperm head and middle piece 180° as it veers from

the penetration path to the copulation path, in line with the egg nucleus. (E)

First division spindle forming at right angles to the copulation path. Since the

copulation and penetration paths are not in the same plane, the cleavage furrow

does not cut through the middle of the gray crescent. (F) When the penetration

and copulation paths are continuous and in essentially a straight line, the first

cleavage furrow will bisect the gray crescent.

Immediately upon invasion by the spermatozoon tiie egg substance

becomes more labile and there is a streaming of the protoplasm toward

the animal pole and of the yolk (deutoplasm) toward the vegetal pole.

Polarity is therefore accentuated. The movement of the egg contents

is reflected in the very violent activity of the microscopic pigment

granules on the surface. These granules appear motionless in the un-

fertilized or the dead egg. These facts have been established by cyto-

logical and experimental investigations, and observed in high mag-

nification motion pictures. However, since the sperm penetration


involves the loss of pigment from the surface of the egg, there appears

a compensatory marginal region between the animal and the vegetal

hemispheres which is neither black nor white, but is gray. This inter-

mediate shading is due to the partial loss of surface pigment and the

consequent greater exposure of the underlying yolk, so that it is known

as the gray crescent. It establishes the gray crescent plane, and is

known as the plane of embryonic symmetry. The gray area has a

crescentic shape, due to the spherical surface involved. The surface

pigment is largely in a separate non-living coat, but one which is

necessary for the maintenance of cell integrity and is actively involved

in the subsequent cleavage process. But this sheet of pigment is pulled

slightly in the direction of the penetrating sperm which carries part of

the pigment into the egg. The opposite side of the sheet of pigment

is therefore pulled away from the underlying yolk, leaving it partially

exposed. The gray crescent is entirely a surface phenomenon and has

nothing whatever to do with syngamy (the fusion of gametes). Even

if the sperm aster formation is experimentally inhibited, the gray

crescent will form, it being a response to sperm penetration. The region

of the gray crescent will become the posterior side, and the opposite

(region of sperm entrance) will become the anterior side of the future

embryo. If we now take any two points on the egg axis, and the center

of the gray crescent as a third point, this plane represents the median

sagittal plane of the future embryo. It must be pointed out that the gray

crescent is not always readily apparent, and yet such eggs will develop

normally.

It is presumed that the closer to the equator that the sperm makes

Formation ol the gray crescent. (A) Egg of Rana pipiens at the moment of

insemination. (B) Same egg 20 minutes later showing the iormed gray crescent.

SYMMETRY OF THE EGG, ZYGOTE, AND FUTURE EMBRYO 85

its entrance into the egg, the more extensive will be the opposite gray

crescent. And the nearer it is to the animal pole the less apparent will

the gray crescent be. This gray crescent appears in about one hour

after insemination. It may develop earlier and be more extensive in

aged eggs. While it will move somewhat, by the process of epiboly

(e.g., downgrowth of surface materials), it will ultimately be found

in the region of the blastopore and the anus. This is the region of origin

of most of the mesoderm. It can be seen readily with the naked eye or

under low magnification, but it is difficult to detect in the sectioned

egg because the pigmented layer is relatively very thin. These de-

scriptions of egg polarity and rotatory symmetry, and of the secondarily

imposed bilateral symmetry, are of fundamental importance in under-

standing the development of the normal embryo. However, factors of

unequal pressure on the egg in any clutch of eggs may so alter the

position of the nucleus and the distribution of yolk and or cytoplasm

that the bilateral symmetry of the fertilized egg and the early cleavage

planes may not be so causally related. Even so, normal embryos will

develop.

CHAPTER FIVE

Cleavade

Determination of Planes

Cleavage LawsSteps in the Cleavage Process

Determination of Planes.

The plane of the first cleavage is determined in a very different

manner. The cytoplasm within which the cleavage is initiated is some-

what flattened beneath the pigmented cortex of the animal hemi-

sphere. The egg nucleus lies within this cytoplasm, and, as the sperm

nucleus approaches it, preceded by its divided centrosome, an amphi-

aster is set up to divide the chromatic material of the two syngamic

nuclei. The cleavage plane therefore is determined by the direction of

approach of the sperm nucleus, or rather, the copulation path. It can

be assumed that in the undisturbed egg the cytoplasm also has a slight

axial distribution, paralleling that of the yolk, with its maximum in

the opposite direction to the yolk. The cleavage furrow will form at

right angles to the amphiaster, and this coincides exactly with the

sperm copulation path. One may conclude, therefore, that under un-

disturbed conditions the first cleavage furrow includes (or is de-

termined by) the copulation path.

It should be made clear at this point, however, that the cleavage

plane need not necessarily represent the median longitudinal axis

(sagittal plane) of the embryo, cutting it into right and left halves.

Should this happen it would mean that the penetration and the copula-

tion paths were in exactly the same plane and the first cleavage would

represent a median sagittal plane of the future embryo. Further, the

mere pressure from other eggs in the same clutch can so alter the dis-

tribution of cytoplasm that the cleavage spindle may be shifted from

its expected position. The median plane of the bilaterally symmetrical

embryo may, but does not necessarily, coincide with the plane of egg

symmetry.

To summarize, the penetration point combined with any two points

on the egg axis establishes a plane which represents the median sagit-

87

88 CLEAVAGE

tal plane of the future embryo. The copulation (syngamic) path of the

sperm nucleus to the egg nucleus determines the first cleavage plane.

Should the penetration and copulation paths be in a direct line to the

egg nucleus, the first cleavage would bisect the future embryo ( and the

gray crescent) into right and left halves. There is a natural tendency

for the coinciding of the gravitational plane, the sperm entrance point,

the sperm penetration path, the median plane of the egg, the median

plane of the embryo, and the plane of the first cleavage. But this is not

essential to the production of a normal embryo.

Cleavage Laws.

There are four major cleavage laws, known by the names of those

who first emphasized their importance.

1. Pfluger's Law, The spindle elongates in the direction of least

resistance.

2. Balfour's Law. The rate of cleavage tends to be governed by

the inverse ratio of the amount of yolk present, in holoblastic cleav-

age. The yolk tends to impede division of both the nucleus and the

cytoplasm.

3. Sack's Law. Cells tend to divide into equal parts and each newplane of division tends to bisect the previous plane at right angles.

4. Hertwig's Law. The nucleus and its spindle generally are found

in the center of the active protoplasm and the axis of any division

spindle lies in the longest axis of the protoplasmic mass. Divisions tend

to cut the protoplasmic masses at right angles to their axes.

Steps in the Cleavage Process.

The frog's egg is telolecithal, meaning that there is a large amount

of yolk concentrated at one pole, opposite to the concentration of

cytoplasm and the location of the nucleus. The cleavages are holo-

blastic (i.e., total), and, after the second cleavage, they are unequal.

They eventually cut through the entire egg from the surface inward.

About 2Vi hours after the egg is fertilized there appears a very short

inverted fold near the center of the pigmented and slightly depressed

animal hemisphere cortex. This depression is extended in both direc-

tions and in the pigmented surface coat there appear lines radiating

outwardly from the deepening first cleavage furrow. It appears as

though some internal force is drawing this cortical region (surface)

of the egg toward its center, and the apparent tension lines may be the

CLEAVAGE 89

result of these inner pulling forces. The facts that the radiating "ten-

sion" lines are not always developed, that they vary in different eggs,

and that they can be chemically obliterated without interfering with

the cleavage indicate that they are not the forces involved in cleavage

but that they result from such forces. As the furrow is deepened and

becomes fully formed, these lines disappear, and the surface of the egg

The first cleavage. Tension lines in the cortex during

furrowing.

again becomes smooth. Similar though less pronounced lines gen-

erally are seen in association with the second and subsequent cleav-

ages.

The furrow is at first very shallow (superficial), extending from the

center of the animal hemisphere around the egg toward the vegetal

hemisphere. Its greatest depth is where it is first seen, in the animal

hemisphere. Within about 3 hours after fertilization the entire egg is

ringed by a vertical furrow. This is always slightly deeper at the ani-

mal than at the vegetal hemisphere, and is clearly marked by the

concentration of surface pigment. Such grading of the furrow may be

due to the mechanical factor of the more resistant yolk at the vegetal

pole. In most, but not in all, cases the first cleavage cuts through the

90 CLEAVAGE

gray crescent. After the first cleavage and up to the late blastula stage,

each cell of the embryo is known as a blastomere.

Before the first cleavage furrow has completely encircled the egg, a

second furrow begins at the center of the animal hemisphere and at

right angles to the first furrow. This cleavage is also vertical and will

progress down and around the egg to become deeper and deeper, but

always deepest at the animal hemisphere. This second cleavage first

begins to appear at about SVi hours after fertilization, or 1 hour after

the completion of the first cleavage. Again the so-called tension lines

appear, and, since the lines of the first furrow have by this time flat-

The 4-cell stage. (A) Animal pole view, to show second

cleavage at right angles to the first. (B) Lateral view of

the 4-cell stage showing incomplete encirclement of the

vegetal hemisphere of the second cleavage furrow.

tened out, the presence of these new lines plus the relative shallowness

of the furrow will help in its identification as the second furrow. As

in the case of the first, this second furrow progresses superficially

around the egg until the two ends meet at the vegetal pole. In the

meantime the first cleavage furrow has cut more deeply into the egg

so that in transverse (horizontal) sections of such an egg one can

identify the two cleavage furrows easily by their relative depth.

We can picture the egg at this stage as a core of yolk, concentrated

more toward the vegetal pole, and with the cytoplasm, nuclei, and

deepest portions of the cleavage furrows located at the pigmented

animal hemisphere. However, all of the animal hemisphere structures

will spread progressively toward the vegetal pole. With each division

of the cytoplasm, cells are formed which contain yolk. By the time

the third cleavage makes its appearance, the first cleavage has all but

cut the entire egg mass into two equal parts. Each of these first two

blastomeres (and the subsequent four blastomeres) is identical with

all of the others with respect to the cytoplasm, pigment, and yolk

CLEAVAGE 9

1

gradients. In the 4-cell stage they are not qualitatively identical, for

only two of the four cells contain any of the gray crescent material.

Often in closely packed eggs these first two cleavages may appear

not quite in the center of the animal hemisphere, or may be not at

right angles to each other. This would be the exception, however, and

generally is due to the physical compression of the closely packed eggs

whereby (as pointed out by Sachs and Hertwig) the cytoplasmic axis

is shifted. The description given here applies to the isolated and un-

restricted egg and should be considered as the more nearly normal.

However, it must be pointed out that this minor compression and

shifting of cleavage planes does not affect the normal development of

the embryo, provided the distortion is not too great.

The third cleavage begins about 30 minutes after the second is com-

pleted, or 4 hours after fertilization. The cleavage rate is accelerated

with each division, possibly because the resulting blastomeres are

progressively smaller. The third cleavage plane is horizontal (lati-

tudinal ) and at right angles to both the first and the second cleavages.

It is slightly above the level of the equator, about 60° to 70° from the

center of the animal hemisphere. This shifting of the cleavage plane

toward the animal hemisphere from the equator of the otherwise

equatorial cleavage is due, no doubt, to the presence of more cytoplasm

in that region and to relatively less yolk. Yolk is generally resistant

to cleavage forces. When the cleavages are completed, the uppermost

and smaller cells are sometimes designated as micromeres (smaller

blastomeres) and the lowermost and larger cells are then designated as

macromeres (larger blastomeres). This third cleavage furrow is seen

as a horizontal groove varying from 20° to 30° above the equatorial

line, toward the animal hemisphere. It provides four clear-cut blas-

tomeres in the animal hemisphere, each equally pigmented. Since the

second cleavage may not have been completed at the vegetal pole,

there is a brief transitional period in which from some aspects there

appear to be the surface markings of 6 rather than 8 cells.

The fourth cleavage is usually double and occurs about 20 minutes

after the third. It tends to be vertical, but is seldom meridional. Thefurrows begin near the center of the animal pole and progress vegetally,

dividing each of the original four blastomeres of the animal hemi-

sphere. Instead of intersecting at the center of the animal hemisphere

the fourth cleavage furrows may run into the first or the second

cleavage furrows. This cleavage shows the first digression from a regu-

92 CLEAVAGE

The third, fourth, and fifth cleavages from the animal pole and lateral views.

lar pattern in the undisturbed egg, and may appear even as a well-

defined furrow before the third (horizontal) cleavage has reached its

maximum depth. The cleavage rate is accelerated with each of the

early divisions, and, since the blastomeres are of unequal size and

have varying amounts of cytoplasm and yolk, synchronous cleavage is

lost and there is an obvious overlapping of the divisions. The upper-

most cells (micromeres) divide the most rapidly so that for a period

there may be a 12-cell stage, made up of 8 upper and 4 lower blas-

tomeres. Perfect symmetry in cleavage and in the blastomeres from

this point on is a rarity, and yet the embryo will develop perfectly

normally.

The fifth cleavage (also double), which should provide a 32-cell

embryo, is more apt to be out of line, and the symmetry and regularity

of the earlier cleavages is lost. The gradient of cleavage rate from the

rapidly dividing animal hemisphere blastomeres to the slowly dividing

vegetal hemisphere cells now becomes more apparent. These newfurrows tend to be horizontal (latitudinal), as was the third cleavage.

One cuts the animal hemisphere blastomeres and the other the vegetal

hemisphere blastomeres horizontally, so that tiers of blastomeres are

formed. The smallest cells, which contain the most pigment, are nowfound at the animal pole, and the largest blastomeres with no pigment

and the most yolk are at the vegetal pole. There is evident, however,

some movement of the surface (cortical) pigment in the direction of

the vegetal pole accompanying each division of the blastomeres. This

process is known as epiboly, to be described later in this book.

CHAPTER SIX

Blastulation

Internally the 8 -cell stage of the frog shows the beginning of a

cavity where the well-formed animal hemisphere cells have completed

their 6-sided cell membrane. Once a blastomere is formed it tends to

acquire a certain degree of rigidity and will maintain its spherical cell

boundaries independently of its neighbors. Cortical and vitelline mem-brane pressure may flatten cells slightly against each other. When seg-

mentation of the egg occurs, dividing it into smaller and smaller

cellular units, there naturally follows the appearance of an internal

cavity known as the segmentation cavity or blastocoel. The aggregate

becomes about 20 per cent larger than the total volume of the cells.

The earlier cleavages (described above) tend to be perpendicular to

the egg surface. However, after the 32-cell stage there appear division

planes more or less parallel to the egg surface, cutting off surface

protoplasm from inner protoplasm and yolk. With increased cell

rigidity, following each division, these surface cells push against each

other until they are lifted up and away from the underlying cells. Acrude simile would be to hold four or more marbles tightly together in

the hand, and no matter how much pressure is exerted there will always

be a space in the center of the group of marbles (i.e., cells). In the

frog's egg the formation of complete cells is rather slow, and the

blastocoel is small and not readily apparent until about the 32-cell

stage.

The blastocoel is therefore a cavity, and this stage of development

is known as the blastula, overlapping the stages of cleavage. The cavity

is enlarged with each of the early cleavages and it is filled with an

albuminous fluid, arising from the surrounding cells. Even though the

frog's egg is telolecithal, cleavage has been described as holoblastic

(opposed to meroblastic) and the presence of this cavity distinguishes

it as a coeloblastula (as opposed to stereoblastula which has nocavity). Since the horizontal cleavages appear toward the animal

hemisphere, this newly forming blastocoelic cavity will appear in an

eccentric position above the level of the equator, and slightly toward

93

94 BLASTULATION

Animal hemispheres^ Segmentotion cavity (blastocoel)

~ Vegetal hemisphere

Hemisected blastula

Segmentotion covity (blastocoel)

Lote biostula Hemisected late blaslulo

Blastulation in the frog. (Redrawn and modified after Huettner.)

Vegetal hemisphere

Frog blastula: sagittal section.

BLASTULATION 95

Progressive stages in blastulation.

the gray crescent side of the cleaving egg. It will remain in this position,

beneath the animal hemisphere, until it is later displaced by the de-

velopment of other cavities. The size of the blastocoel increases with

the formation of smaller and smaller surrounding cells. The late blas-

tula of the frog, by virtue of this blastocoel, has a volume displace-

ment about 20 per cent greater than that of the fertilized egg.

Following the 32-cell stage the egg loses all semblance of rhythmic

or synchronized cleavage and there is developed a gradient of cleavage

with the most active region being at the animal pole and the least

active being at the vegetal pole. In addition to this there are some

characteristic changes in the biastula of the frog embryo. First, the

smaller pigmented animal pole cells tend to spread their activity in a

downward direction toward the vegetal pole, by a sort of contagion,

so that there is an actual migration of pigment-covered cells toward

the vegetal pole. This, and the horizontal cleavages, result in increas-

ing the cell layers but thinning of the roof of the blastocoel. Second,

the horizontal cleavages in the very small animal pole cells give the

blastula a double or multi-layered roof. The single outer layer of cells

contains most of the superficial pigment and is now recognized as the

epidermal layer which will give rise to epithelium, either of the integu-

ment or lining the nervous system. The inner tiers of cells of the

blastular roof are less pigmented and are known collectively as the

nervous layer because they will give rise largely to the neuroblasts of

the nervous system.

96 BLASTULATION

Either on the surface of the whole blastula or in sections of the

blastula, one can determine the margins of the downward-moving

pigmented coat and active animal hemisphere cells and the conse-

quent slight thickening of the equatorial band. This margin is knownas the marginal zone, marginal belt, or germ ring. Such a marginal

region of activity, where yolk is most actively being transformed into

cytoplasm, is found in Amphioxus and chick, and probably in all

vertebrate embryos. It will have much to do with the subsequent

formation of the lips of the future blastopore.

The marginal zone is approximately equatorial in the blastula

stage, except for the slight reduction in pigmented cells at the side

where the original gray crescent was located. Opposite to this gray

crescent region the wall of the blastula appears to contain relatively

more layers of cells and is therefore somewhat thicker. These changes

occur in anticipation of the next process in embryonic development,

namely gastrulation.

There have been numerous attempts to explain blastocoel forma-

tion and the movements anticipating gastrulation. These have been

based largely on purely physical phenomena. An attempt has been

made recently to summarize these concepts, as follows (Holtfreter,

1947: Jour. Morph., 80:42):

If present in large numbers, the cells at the periphery of the body tend

to establish a semipermeable layer, while the cells of the interior becomeseparated from each other by secretion fluid. Thus the aggregation may be

transformed into the configuration of a blastula or, if the amount of internal

fluid increases further, into an epithelial vesicle. The occurrence of cylin-

drical stretchings and of migrations of the peripheral cells into the interior

of the vesicle has been interpreted as possibly being caused by a surface

tension lowering action of the inner fluid. Even in the absence of a gradient

of surface tension, however, isolated embryonic cells tend to elongate and

to migrate in one direction, with their original proximal pole leading the

way, and possibly invagination movements may be partly due to this in-

herent dynamic tendency of the individual cell. Other factors influencing

shape and movements of the cells in aggregate, or in a normal embryo,

appear to be cell-specific differences of adhesiveness arising in the course

of differentiation and making for a sorting out, grouping and aggregation

of the various ceil types into tissues and organs. With the appearance of

fibrous and skeletal structures, of local differences in mechanical stress and

hydrostatic pressure, of differential growth and a variety of metabolic proc-

esses, so many new factors are introduced that the principle of interfacial

tension loses more and more of its original primacy as a morphogenetic

agent.

CHAPTER SEVEN

Gastrulation

Significance of the Germ Layers Pre-gastrulation Stages

Origin of Fate Maps Definition of the Major Processes of

Gastrulation as a Critical Stage in Gastrulation

Development Gastrulation Proper

Gastrulation is that dynamic process in early development which

invariably results in the transformation of a single-layered blastula

stage to a didermic or 2-layered embryo. It involves, but is inde-

pendent of, mitosis. The process varies considerably among the Verte-

brates, but to a lesser extent when the process is compared in closely

related species. The two layers to be distinguished are the ectoderm

and the endoderm. In many forms (e.g., the frog) the third germ layer,

or mesoderm, is formed almost simultaneously with the endoderm.

Significance of the Germ Layers

The distinction of germ layers, such as the ectoderm, mesoderm,

and endoderm, is purely a matter of human convenience and is of no

real concern to the embryo. The mere fact that the endoderm and the

mesoderm are both derived originally from the ectoblast (outermost

layer of the blastula) suggests their fundamental similarity. By means

of experimental procedures it is possible to demonstrate that these

presumptive germ layers are interchangeable and that regions ordi-

narily destined to become, for example, brain (ectoderm), may be

transplanted to another region of the blastula where they will become

muscle (mesoderm) or possibly thyroid (endoderm). The germ layer

distinctions are based upon their position in the developing organism,

and their fate. The ultimate tissues of the adult are often classified,

again as a matter of human convenience, on the basis of groups which

are derived from one or another of these three primary germ layers.

The germ layers, which are first distinguishable with gastrulation,

therefore are definable by their fate in embryonic development. How-ever, this fate is an arbitrary distinction since the fate of any group of

cells can be altered by transplantation. The importance, then, of this

97

98 GASTRULATION

distinction is one of position, and the ectoderm is always the outer-

most layer of cells, the endoderm the innermost, and the mesoderm

the intermediate layer of cells present when gastrulation has been com-

pleted.

In recapitulation, a histologist could define the ectoderm as that

group of embryonic cells which, under normal conditions of develop-

ment, give rise to the epidermis and the epidermal structures, to the

entire nervous system and sense organs, to the stomodeum, and to the

proctodeum. The endoderm would be defined as that group of em-

bryonic cells which normally give rise to the lining epithelium of the

entire alimentary tract and all of its outgrowths, such as the thyroid,

lungs, liver, pancreas, etc. The mesoderm would be defined as that

group of cells which normally give rise to the skeleton and connective

tissues, muscles, blood and vascular systems, to the coelomic epi-

thelium and its derivatives, and to most of the urogenital system. The

notochord in the frog is derived simultaneously with the mesoderm,

and probably from the dorsal lip epiblast.

It is important, therefore, for the student to avoid ascribing any

peculiar powers to the germ layers as such. They have their particular

fate in development not by virtue of any peculiarly endowed power

but rather by their position in the developing embryo as a whole.

Origin of Fate Maps

Gastrulation is a critical stage in the development of any embryo.

This is due in part to the fact that the positional relationship of the

various cells of the late blastula and early gastrula begins to take on

special significance. This has been demonstrated by many experimental

embryologists, among whom are His, Born, BUtschli, Rhumbler, Spek,

Vogt, Dalcq, Pasteels, Vintemberger, Holtfreter, Nicholas, and

Schechtman. They have used various experimental devices, such as

injury or excision of cell areas, or staining local areas with vital dyes

and following their subsequent movement. By such methods the so-

called "fate maps" of various blastulae have been worked out.

A fate map is simply a topographical surface mapping of the blastula

with respect to the ultimate fate of the various areas. When one traces

a cell group on the surface of a blastula which has been vitally stained

with Nile blue sulfate, for instance, and finds that these cells movefrom the marginal zone (between the animal and the vegetal hemi-

spheres) over the dorsal lip of the early blastopore and into the embryo.

ORIGIN OF FATE MAPS 99

100 GASTRULATION

where they become pharyngeal endoderm, he is then able to label the

fate of that area on other blastulae. The fate maps of various amphibiaare basically alike, but they are not sufficiently alike in detail to per-

mit plotting a universal amphibian fate map. The frog's egg is very darkand it is difficult to apply vital dyes so that they will be visible on the

Morphogenetic movements during gastrulation and neurulation as indicated by

changes in position of applied vital dyes.

GASTRULATION AS A CRITICAL STAGE IN DEVELOPMENT 101

Epidermis

SuckerSucker

Epidermis

Lens

Neural fold

Ear

Limit of head region

Limit of inturnedmaterial

Notochord

Mesoderm, somites

Anterior limb bud

Blastopore lip

Visceral pouches

POSTERIOR- DORSALVIEW

RIGHT LATERALVIEW

Fate map showing presumptive regions of the anuran blastula. (Adapted from

Vogt: 1929.)

frog blastula. However, the fate maps of closely related species have

been worked out and, combined with circumstantial experimental

evidence on the frog's egg, we are able to supply what is believed to be

a reasonably accurate fate map of the frog blastula.

Under conditions of normal development, the ectoderm of the frog

is derived in part from the cells of the animal hemisphere and in part

from the intermediate or equatorial plate cells. These are the regions

on the fate map designated as presumptive ectoderm. The endoderm

comes partly from the intermediate zone but largely from the vegetal

hemisphere area (i.e., the presumptive endoderm on the fate map).

The mesoderm and the notochord have a dual origin, arising between

the ectoderm and the endoderm, largely from the region known as the

lips of the blastopore. The student is advised to study carefully the

accompanying fate maps.

Gastrulation as a Critical Stage in Development

One of the reasons for the fact that gastrulation is such a critical

stage in embryonic development is that not only do the cells divide but

also various cell areas take on special significance which they did not

previously have. This special significance is shown by the mere fact

that fate maps have been derived. There is a new independence, as

well as interdependence, of various areas of the embryo. This change is

designated as differentiation. Instead of a group of somewhat similar

102 GASTRULATION

cells, arranged more or less in a sphere, each cell having essentially the

same potentialities as any other cell, there is now a mosaic of cell

groups of integrated differences. The differences in the various areas

may not be apparent physically, but they can be demonstrated func-

tionally and constitute the process of differentiation. This is probably

the most critical change that is brought about by gastrulation.

Another reason for the critical nature of gastrulation lies in the

mechanics of the process itself. There is a localized interruption in the

Gastrulation in the frog, Rana pipiens (photographs). (A) Initial involution

at the dorsal lip. (B) Crescent-shaped lips of the blastopore extending laterally

along the margin of the gerrn wall. (C) Half-moon-shaped involuted dorsal and

lateral lips of the blastopore. Presumptive areas or organ anlagen indicated.

(D) Lips of the blastopore completely encircle the exposed yolk plug. Rotation

of the entire embryo through 90°,

PRE-GASTRULATION STAGES 103

continuity of the epibolic movement of the surface coat toward the

vegetal hemisphere. This is only an apparent interruption, since it in-

volves an inturning of cells at a very specific region of the marginal

zone, or the most ventral limit of the pigmented animal hemisphere

cells. This specific region is the ventral limit of the original gray cres-

cent, destined (at the time of fertilization) to become the region of

formation of the dorsal lip of the blastopore, with all of its implica-

tions.

Cells which lie on the lateral surface of the late blastula begin to

roll inwardly, first only a few cells and then, by a sort of contagion, the

contiguous cells of the more lateral marginal zone. It is this infolding

process which marks the actual, observable process of gastrulation. It

must be emphasized that these observable changes are probably long

anticipated, as will be suggested by the detailed description below. If

there is interference with this inturning movement of cells, any sub-

sequent development is apt to be abnormal or incomplete if, in fact,

it occurs at all. Embryos at the time of gastrulation are indeed hyper-

sensitive to physical changes in the environment, and to genetic in-

compatibilities within the chromosomes of the involuting cells. Em-bryos which survive this process may reasonably be expected to

achieve the next major step, namely neurulation.

Pre-gastrulation Stages

There is no clear-cut demarcation between the blastula and the

gastrula stages, unless one accepts the initial involution of the dorsal

lip cells. Some investigators have pointed out that there is a dispro-

portionate ratio of the yolk and cytoplasm to the nucleus of the fer-

tilized egg, as compared with the ratio in the somatic cell. This sug-

gests to them that cleavage results in a progressive approach to the

somatic ratio of nuclear volume to cytoplasmic volume. When the

ratio is reached whereby the nucleus can properly control its cyto-

plasmic sphere of influence, then the latent influence can begin to exert

itself and the process of differentiation begins. Such a situation occurs

at the beginning of the gastrula stage. Cleavage continues, of course,

but it has been proved beyond a doubt that cell areas are no longer

of equivalent potency with regard to ultimate development. It has been

suggested, therefore, that the blastula stage is monodermic (one prin-

cipal outer layer of cells, generally arranged somewhat spherically)

and contains a blastocoel. The end of the blastula stage is reached

104 GASTRULATION

when the nuclear to cytoplasmic volume relationship of the con-

stituent cells becomes most efficient, differentiation begins, and we can

observe the movements and surface changes attendant upon gastrula-

tion.

Until recently the yolk hemisphere of the early cleavage stages of

the amphibian embryo was considered to be relatively inert. It was

even considered a deterrent to morphogenetic movements of gastrula-

tion. The vegetal hemisphere becomes cellular, but more slowly than

does the animal hemisphere. The cells are larger and always contain

abundant yolk. Nicholas (1945) wrote: "The concept of the inert-

ness of the yolk mass probably inhibited our realization of its possible

import." This attitude is now subject to change.

By applying vital dyes to the yolk hemisphere of the early and late

blastula stages, Nicholas has found positive activity on the part of these

yolk-laden vegetal pole cells, suggesting that they are concerned with

the formation of the blastocoel, with the process of ingression, and

also with the changes in water balance and later rotation. It nowseems that the yolk assumes a dynamic rather than a passive role both

in the process of blastulation and in anticipating the changes pre-

requisite to gastrulation.

In an exhaustive series of studies, Holtfreter and Schechtman have

attempted independently to analyze the morphogenetic movements

both before and during the gastrulation process in the amphibia. It is

very probable that the formative influences so evident at the time of

gastrulation are present long before the initial involution of cells to

form the dorsal lip of the blastopore. The following description is

based largely on the studies of these investigators and is supported by

direct observation, available to anyone.

There is a protein-like surface coating or film on amphibian eggs,

present from the beginning. It has plastic elasticity, is not sticky on

its outer surface, but is an integral syncytial part of the living egg. This

coat persists and its strength increases during development. It is in-

volved in matters of cellular elasticity, cell aggregation, cell polarity,

cell permeability, resistance to external media, osmotic regulation, and

tissue affinity—all physical phenomena which are important in gas-

trulation movements.

This surface coating is divided only superficially by the segmenting

blastomeres of the early embryo and yet (Holtfreter): "the syncytial

coat thus represents at least one, and perhaps the most significant one.

PRE-GASTRULATION STAGES 105

of the effective forces that make for a morphogenetic integration of

the dynamic functions of the single cells into a cooperative unity."

Embryologists have long been in search of a controlling supercellular

physical force which might explain at least the initiation of the move-

ments leading to gastrulation. This surface coat of Holtfreter may well

be that controlling and integrating force. He suggests further that

probably all of the non-adhesive epithelial linings of the larva can

be traced back to this original surface coat.

(kjina pipiens) (stags identified by number)

Blastula to tail bud stages. (AP) Animal pole. (BR 1) First branchial cleft.

(BR 2) Second branchial cleft. (BR 4) Fourth branchial cleft. (DLBP) Dorsal

blastopore lip. (GA) Gill anlage. (GP) Gill plate. (GR) Germ ring. (HMCL)Hyomandibular cleft. (LNF) Lateral neural fold. (MP) Medullary plate. (MY)Myotome. (NG) Neural groove. (OP) Optic vesicle. (OS) Oral sucker. (PRN)Pronephric region. (SP) Sense plate. (TNF) Transverse neural fold. (VP)Vegetal pole. (VLBP) Ventral blastopore lip. (YP) Yolk plug.

106 GASTRULATION

{Top) Tritmus torosus, gastrula. (a) Surface view of the blastoporal region

after part of the surface layer has been removed, (b) Isolated bunch of flask

cells from the lateral blastoporus lip, containing larger endodermal and smaller

mesodermal cells. {Center) A. punctatum, gastrula. Surface view of the blasto-


DEFINITION OF THE MAJOR PROCESSES OF GASTRULATION 107

The blastula stage is held to a spherical aggregate by the presence

of this surface coating. However, since the coating is not itself divided

with each cleavage, but simply folds into the furrows between blas-

tomeres, the inner boundaries of the cells do not have the same domi-

nating force that is present on the outer surface. Such cells are found

to be interconnected by slender and temporary processes. The forma-

tion of cells, once thought to be the cause of gastrular movements, is

now thought to be merely a convenience for the execution of such

movements.

Epiboly in the frog is the concern of this surface coat rather than

of individual cells. It is due to the expansive nature of the surface

coating while it is in contact with a suitable substratum. The potential

ectoderm (animal hemisphere cells) and the potential endoderm

(vegetal hemisphere cells) are, in fact, competitors in the tendency to

cover the surface area. The driving force is this surface coat, and for

some reason the potential of the animal hemisphere portion of it is

greater than the potential of the surface coat of the vegetal hemisphere.

Definition of the Major Processes of Gastrulation

Gastrulation is now recognized as an extremely complicated but

highly integrated and dynamic change in the embryo, brought about

by a combination of physical and chemical forces arising intrinsically

but subject to extrinsic factors. We do not yet fully understand the

process in any animal, particularly because of the elusive nature of the

forces involved. Gastrulation is concerned with cell movements,

changes in physical tension, and in the metabolism of carbohydrates,

proteins, and possibly even the lipids.

Before attempting to describe the process as it occurs in the frog,

it would be well for us to have a clear-cut understanding of the mean-

ing of the various terms often encountered in such a description. The

student is advised to consult frequently the Glossary at the end of this

text, not only while reading this section but also throughout the dis-

cussion of embryology as a scientific subject.


pore showing the accumulation of pigment and the stretching of the cells toward

the lines of invagination. (Bottom) A. piinctatum, blastula. Section of the pro-

spective blastoporal region. Note the black streaks of condensed coat material,

and the flask cells attached to it. (Courtesy, Holtfreter, 1943, J. Exper. ZooL,

94:261.)

108 GASTRULATION

Invagination—As used by Vogt, this term means insinking (Ein-

stulping in German) of the egg surface followed by the forward

migration (Vordringen) which involves the displacement of inner

materials. Schechtman uses the term to mean an inward movement,

without any reference to whether there is pulling, pushing, or autono-

mous movement. There is probably some invagination in gastrulation

of the frog's egg.

Involution—As used by Vogt, this term means a turning inward,

a rotation of material upon itself so that the movement is directed

toward the interior of the egg. Involution does occur in the frog's egg

gastrulation.

Delamination—This is a splitting-oflf process whereby the outer

layer of ectoderm gives rise to an inner sheet of cells known as the

endoderm. It does not occur in the frog's egg gastrulation.

Epiboly—This is a progressive extension of the cortical layer of

the animal hemisphere toward the vegetal hemisphere, or a sort of ex-

pansion from animal toward the vegetal hemisphere. In some eggs

(e.g., the mollusk Crepidula) it actually involves an overgrowth by

smaller cells over the larger vegetal hemisphere cells. The extension

type of epiboly does occur in the frog's egg gastrulation.

Extension—This term is used by Schechtman to describe in am-

phibia the self-stretching process which seems intrinsic within certain

cell areas, particularly in the dorsal region of the marginal zone. It is

Diagrams to show the directions of movement and the displacement of the

parts of the blastula in the process of gastrulation in amphibia. (After W. Vogt,

1929b. From Spemann: "Embryonic Development and Induction," New Haven,

Yale University Press.)

GASTRULATION PROPER 109

Presumptive

neural plate

Lateral / Lips of blastopore

Involutional movements during gastrulation outlined on the living gastrula.

synonymous with elongation (or Strekung of Vogt), and probably

occurs in the frog's egg gastrulation.

Constriction—This term is used to describe the convergence of cell

areas resulting in the gradual closure of the blastopore. It is due to a

narrowing of the marginal zone and a pull, or tension, of the dorsal lip.

This occurs in the frog's egg gastrulation.

Dorsal convergence—This is the dorsal Raffung of Vogt, used to

describe the movement of the marginal zone cell areas toward the

dorsal mid-line during involution and invagination.

With this general introduction to a very fascinating but, as yet, litde

understood phase of embryological development, we shall retrace the

steps to the late blastula stage and endeavor to anticipate the process

of gastrulation in the frog.

Gastrulation Proper

To understand the processes to be described as gastrulation, let us

list in succession the changes that may be observed in the frog embryo.

1. Thinning of the gray crescent side of the blastula wall. There

appears to be an actual migration of cells from the deeper layers

no

Germ ring

GASTRULATION

Epithelial ectoderm

Blastocoelastocoel

Side of the gray

crescent, blasto-

pore, onus, andmesoderm forma-tion

Germ ring

Dorsal

Initial invagination

at dorsal lip

Dorsal

Margin of successive

involuted lip cells

Portion of gray

•\ crescent

Margin of germ ring

Ectoderm Endoderm

horda-mesoderm

Dorsal blastopore lip

Blastopore

PosteriorYolk plug

Ventral blastopore lip

esoderm

Ventral

Notochord

Ectoderm

Margin of ectoderm

Anterior margin of

mesoderm

Mesoderm

Chorda -mesoderm

Blastopore

The process of gastrulation. (A) The blastula stage, prior to any gastrulation

movement. (B) Movement of the blastula cells preliminary to gastrulation.

(Continued on facing page.)

GASTRULATION PROPER

Animal pole

111

Sperm entrancepoint

Ventral

side \ io»|

30° \

Limit of involution

L Dorsalr side



Gray crescent

Vegetal pole

Surface changes during gastrulation. (Modified from Pasteels.)

of the blastula away from the original site of the gray crescent.

This is the region where involution will first take place, the

region which will come to be known as the dorsal lip of the

blastopore. The blastula wall which is equatorially opposite to

the gray crescent side may, in consequence, become several

cells thicker.

2. Continued epiboly or extension of the marginal cell zone toward

the vegetal hemisphere , so that the diameter of the marginal

zone becomes progressively smaller as it passes below the equa-

tor. The marginal zone attains increasing rigidity so that it

exerts an inward pressure on the yolk cells which are being en-

circled, causing them to be arched upward toward the blastocoel.

One might draw the simile of an overlapping rubber membranebeing drawn down over a mass of slightly less resistant mate-

rial, toward the center of which there is a cavity. The yolk

{Continued from opposite page.)

(C) Blastoporal view of successive phases of gastrulation; (solid line) lip of

blastopore, {dotted line) germ ring, to be subsequently incorporated into the

blastoporal lips. (D) Lateral view of sagittal section during late gastrulation

showing the origin of the mesial notochord, and the lateral mesoderm from the

proliferated chorda-mesoderm cells at the dorsal lip. (E) Composite drawing to

illustrate the germ layer relations in the later gastrula of the frog. The medullary

plate (ectoderm) is not indicated; {alternate dots and dashes) notochord, {heavy

stippling) notochord, {sparse stippling) mesoderm, (cellular markings) ecto-

derm.

112 GASTRULATION

endoderm cells toward the gray crescent side become separated

from the epiblast and are forced upward, displacing the blas-

tocoel and reducing its size. This process is known as pseudo-

invagination because there is a degree of "pushing in" of the

vegetal hemisphere cells. It is not true invagination, however, as

one understands the process in such a form as the starfish

embryo. The slit-like space between the yolk endoderm and the

epiblast, continuous with the blastocoel, is sometimes referred to

as the gastrular slit.

3. The initial involution or inturning of a jew cells at the lower

margin of the original gray crescent, followed by the lateral ex-

tension of this involution along the epibolic marginal zone. This

region of initial involution becomes the dorsal lip of the blas-

topore. The marginal zone cells become separated from the more

ventral and lighter colored yolk cells. The first cells to involute

do not lose continuity with their neighbors, but carry with them

the adjacent, contiguous lateral cells of the marginal zone. Theinfolding or inturning process therefore begins at one point but

is continued around the marginal zone.

4. Further epiboly of the entire marginal zone toward the vegetal

hemisphere, apparently interrupted only at the levels of involu-

tion. The inturning cells are rolling under themselves like an on-

coming wave. The margin of the wave continues, through

epiboly, to move toward the vegetal hemisphere but at a rate

which seems slower than that of the marginal zone which has

not yet involuted (i.e., opposite the dorsal lip cells). In other

words, cells are moving inwardly over the dorsal lip of the blas-

topore, but the margin of the lip itself is moving vegetally as a

result of epibolic extension. Some epibolic movement is ab-

sorbed in involution.

5. The piling up or confluence of animal hemisphere cells, many

of which are destined to move inside over the blastoporal lips.

6. The continued lateral extension of the involuting marginal zone

cells so that there is eventually a circumferential meeting of the

blastoporal lips. These comprise the dorsal, lateral, and ventral

lips of the blastopore, not clearly demarked. The vegetal hemi-

sphere cells, which are now exposed within a ring of involuted

marginal zone (lip) cells, are collectively known as the "yolk

plug." The term yolk plug is used because the cells of the pig-


merited marginal zone (blastoporal lips) are in sharp contrast

with the surrounded vegetal pole yolk-laden cells.

7. The future epiholy of the marginal zone, accompanying invo-

hition at all points, resulting in a continued reduction in the size

of the circumblastoporal lips and of the exposed yolk plug. The

margins of the yolk plug are at first round, then vertically oval,

and finally the lateral lips of the blastopore approach each other

to form a vertical slit as the yolk plug is closed over and disap-

pears within.

8. The origin of the internal or second layer of cells, arising from

the involuted dorsal lip cells and collectively known as the

endoderm. This sheet of cells fans out within the embryo to give

rise very soon to a new cavity, the gastrocoel or archenteron.

Since these inturning cells give rise largely to the roof and lateral

walls of the archenteron, those parts of the cavity will be lined

with somewhat pigmented cells from the original epiblast. There

is a gradation or lessening of this pigment in the archenteric roof

cells as one progresses anteriorly in the gastrula.

9. Simultaneously with the origin of the inner layer of endoderm,

some cells are proliferated o§ into the gastrular slit, between the

roof of the archenteron and the dorsal epiblast, which cells will

become the notochord. Before differentiation these cells are

called chorda-mesoderm. Since the lips of the blastopore are

circular, this proliferated mass of cells also becomes circular.

The more lateral and also the ventral proliferations becomemesoderm. There arises, therefore, the dorsal notochord and the

lateral and ventral mesoderm as a circle of tissue within the

fold of the blastoporal lips, occupying the space between the

epiblast and either the inner endoderm or yolk endoderm.

By this time the student must have realized that gastrulation is a

highly integrated mosaic of motions which are purposeful in that

they achieve the state of the completed gastrula. Spemann aptly wrote:

However harmonious the process of motion may be by which the material

for the chief organs arrives at the final place, and however accurately the

single movements may fit together . . . they are nevertheless no longer

causally connected, at least from the beginning of gastrulation onward.

Rather, each part has already previously had impressed upon it in some wayor other, direction and limitation of movement. The movements are regu-

lated, not in a coarse mechanical manner, through pressure and pull of

simple parts—but they are ordered according to a definite plan. After an

Medullary plate

(neural ectoderm)Epithelial ectoderm

Notochord

Endoderm

Completionbridge

Blastocoel

Chorda-mesoderm

Dorsal blasto-

pore lip

rS#-Yolk plug

Ventral blasto-

pore lip

Gastrular slit

Peristomial mesoderm

Endoderm ^vtf^*""^ 3k

Archenteron Notochord^^;^''

Ectoderm v^.x„.V

BlastocoelChorda- mesoderm^/^;.>^^

Completion bridge

'r'MXi ^

Yolk plug,j5

. -/. 'J^'>;s^T^*<''-*^^«K[»ia^"êsoderm

^^^^:!r^'*^W^'tS£9S^'¥/&^SSr- Ectoderm

Gastrular slit

Gastrulation and mesoderm formation. {Top') Photograph of sagittal section

through the blastopore. (Bottom) Enlarged photograph similar to above illus-

tration. Note the continuity of the ventral ectoderm and peristomial mesoderm.


114

Medullary plate Notochord

Neural ectoderm / / Endodermal cells

Epithelial ectoderm / ///, Pigmented gut roof

Archenteron

Gastrular slit

Completion bridge

Chorda-mesoderm


Blastocoel

Yolk plug


Peristomial mesoderm

Gastrular silt

Ectoderm

Endoderm

Notochord

A'


Yolk plug


Gastrulation and mesoderm formation

—

(Continued). (Top) Drawing to

be compared with top illustration on facing page. (Bottom) Schematic diagram

to show progression, internally, of the mesoderm.

115

116 GASTRULATION

exact patterned arrangement, they take their course according to independ-

ent formative tendencies which originate in the parts themselves. Thus we

find in the gastrula stage a mosaic, a pattern of parts with definite formative

tendencies from which the formative tendencies of the whole must neces-

sarily follow.

Gastrulation, according to Spemann, is an intricately arranged mosaic

of parts possessing autonomous movement potentials which act in

the right way at the right time.

Animal hemisphere

Blastocoel

Vegetal hemisphere Dorsal blastopore lip

Yolk endoderm . . ,

Archenteron

Blastocoe

Archenteron

Completion

bridge

Blastocoel

Endoderm Notochord

Ectoderm

Yolk plug


Gastrol ,,p*<sssaas^sy Lateral

mesoderm / blastopore

Yolk plug ''P

Blastula to gastrula stages in the Irog. Shown by stereograms.

{Continued on facini,' page.)


Dorsal side

Ventral side

i'?/Yolk

plug

Blastopore lip

Ectoderm

esoderm

Endoderm

W-Yolk plug

H


Archenteron

MesenchymeNotochord Medullary plate

Dorsal blasto-

pore lip

Midgut

Gastral mesoderm

Foregut

Mesenchyme

Notochord

Hindgut

Blastopore

Liver diverticulum

Gastral mesoderm

Blastula to gastrula stages in the frog

—

(Continued) Shown by stereograms.

(Modified and redrawn from Huettner.)

The forces which bring about the initial involution or inturning of

cells at a specific point of the marginal zone to form the dorsal lip

of the blastopore have not yet been identified. The nearest approach

to a valid explanation is that of Holtfreter. He says:

The alkalinity of the blastocoel can be assumed to be strong enough to

establish, in cooperation with the suspended protein particles, an efficient

gradient of surface tension between the internal and external medium, as

envisaged by Rhumbler. Those cells which are in interfacial contact with

both media would be primarily affected by it. They will tend to move in the

direction of the surface tension lowering alkaline medium, and to reduce

their contact area with the external medium having a higher surface tension.

Being, however, attached to the peripheral coat they can only stretch along

the gradient, assuming a shape which can be expected to correspond to those

claviform cells which we find in all cases where invagination takes place.

The peipendicular stretching will have to continue as long as the gradient

118 GASTRULATION

Rotation of the amphibian egg in the gravitational field during gastrulation.

(After V. Hamburger and B. Mayer, unpublished. Redrawn from Spemann:

"Embryonic Development and Induction," New Haven, Yale University Press.

)

persists and until the cells have attained a position where their potential

energy is lowered to a least possible value. This movement is, however, con-

ditioned by the cellular plasticity which is restricted. Thanks to the tensile

strength of the cell wall, the attenuated neck portion is subjected to an in-

creasing mechanical stress. The surface yields and is pulled inside in the

form of the archenteron.

Schechtman (1942) calls involution an "insinking" and then ex-

plains what follows, after having demonstrated that various areas of

the pre-gastrula have various types of autonomous movement. He

says:

Gastrulation begins with autonomous movements; the in-sinking of the

presumptive pharyngeal endoderm, the stretching of the marginal zone to-

ward the blastoporal groove, the forward migration of the internally situ-

ated marginal material along the underside of the animal hemisphere. Asthe stretching presumptive chordal region comes to the edge of the blasto-

poral lip, it is progressively carried under by the invagination and involu-


tion of the adjacent portions of the marginal zone. This insures that the

presumptive chorda is in a position to exert a double effect by means of its

extension, for it will not only pull the lateral marginal zones dorsaiward,

but will also carry them forward in a dorsal position. Meanwhile the blasto-

poral lips are constricted, since there is progressive withdrawal of marginal

zone material by the process of dorsaiward convergence somewhat as the

mouth of a purse is constricted when the purse string is pulled. The tend-

ency of constriction is augmented further by the forward migration of the

internal portions of the marginal zone, for this movement also tends to

withdraw material from the region of the blastoporal lips.

In summary, Schechtman believes:

1. The presumptive chorda is invaginated by the inwardly di-

rected tension or pull exerted by the invagination and involu-

tion of the lateral marginal zone, with which it is continuous.

• 2. The lateral marginal zones are then pulled dorsaiward and

inward in the dorsal position by the autonomous stretching

and simultaneous narrowing of the presumptive chorda.

3. The constriction of the blastoporal lips over the yolk mass is

affected by the progressive withdrawal of marginal zone ma-

terial by dorsaiward convergence.

Internally the involuted cells extend anteriorly, away from the

point of involution or the dorsal lip of the blastopore. The inturned

endoderm surrounds the new cavity, which is at first no more than a

slit. The cavity (archenteron) rapidly expands in all directions so

that the blastocoel, originally in a dorsal position, becomes progres-

sively displaced anteriorly or away from the side of the blastopore

formation. The blastocoel is later displaced antero-ventrally by this

expanding archenteron. It also becomes reduced in size until finally

it is found only as the slit between the endoderm and the yolk, referred

to as the gastrular slit. Sometimes there is a remnant of the blastocoel,

separated from the archenteron by a single layer of cells called the

completion bridge. This bridge is of no consequence, and frequently

ruptures to merge the contents of the blastocoel and the gastrocoel.

These movements of involuting cells and expanding endoderm

result in an enlarged archenteron (gastrocoel), entirely lined with

endoderm. As stated, the roof and lateral wall cells are more pig-

mented than the yolk endoderm floor cells of the archenteron. The

opening beneath the dorsal lip of the blastopore, and into the archen-

teron, is called the blastopore. This is an incorrect term, however, for

120 GASTRULATION

Rotation of the amphibian egg during gastrulation, due to the development of

the internal cavity, the archenteron. (Redrawn from Kopsch.)


the pore opens into the gastrocoel and not into the blastocoel. The

blastopore is occluded by the yolk endoderm cells but later opens

into the perivitelline space by the withdrawal of the yolk plug. It will

close ultimately with the development of tail mesoderm.

The reduction of the blastocoel and the enlargement of the archen-

teron together alter the gravitational axis of the entire embryonic

mass, shifting it about 90" away from the position of the blastopore.

The yolk plug, instead of remaining in a ventral position, comes to

lie about the level of the original equator. This all occurs while

the embryo lies free within its fertilization membrane. Further, the

presence of the archenteric roof and the chorda-mesoderm cells

above it cause the overlying ectoderm to thicken and the whole embryo

becomes elongated in an antero-posterior direction, more or less

horizontal to the revised center of gravity. The yolk plug now marks

the posterior region of the future embryo.

It must be emphasized that, during this shift in the gravitational

axis, there has not been a comparable shifting of the internal relation-

ships of the embryo. The blastopore and all related parts of the em-

bryo have been rotated dorsally, within the enveloping fertilization

membrane. The axis of the gastrula is simply altered by the develop-

ment of this new cavity, the archenteron, with its consequent oblitera-

tion of the blastocoel.

CHAPTER EIGHT

Neurulation and Early Orâiioêny

Neurulation

Early OrganogenySurface ChangesVisceral ArchesOrigin of the Proctodeum and Tail

Internal Changes

Neurulation

The axes of the embryo are altered by the development of the

archenteron. The antero-posterior axis is made obvious by the de-

velopment of the neural axis. Either the roof of the archenteron

and/or the notochord induces, in the overlying ectoderm, the forma-

tion of a thickening, limited to the nervous layer. This becomes the

medullary (or neural) plate which extends from the dorsal lip of the

blastopore in an anterior direction as a median band of thickened

ectoderm which widens anteriorly where the brain will develop.

Shortly the more or less parallel lateral margins of the thickened

medullary plate become even more thickened as the lateral folds

or ridges, and are continued anteriorly as the transverse neural fold

or ridge. A longitudinal neural groove or depression appears in the

center of the medullary plate, so that the height of the neural folds

appears to be accentuated. This is the very beginning of the forma-

tion of the central nervous system, induced by the presence beneath

of the archenteric roof and/or the notochord.

The following description covers the period from the time the

medullary plate first thickens until about the time the embryo reaches

the 2.5 mm. stage. At this time the embryo shows ciliary movement

within its jelly capsule. These cilia are lost, except on the tail, by the

time the 1 1 mm. stage is reached.

123

124 NEURULATION AND EARLY ORGANOGENY

Epithelial

ectoderm

Neural fold

Neural groove

Neural crest

Notochord

Neural crest

Epithelial

ectoderm Nerve cord

Neural canal Notochord

Formation of the neural tube.

Early Organogeny

Surface Changes

The oval-shaped gastrula quickly becomes elongated and the

medullary plate provides a slightly elevated (convex) dorsal surface.

This soon changes to a flattened and then a concave upper surface, as

the development of the central nervous system proceeds. At this

time the embryo acquires a distinct head and a body which is ovoid

because of the mass of contained yolk.

The appearance of the thickened and elongating medullary plate

and the subsequent formation of a neural tube is the main causal

factor in the change in shape of the embryo, which follows upon

gastrulation. In fact, it may well be the autonomous powers of

elongation of the presumptive notochord that are responsible for

,Blostocoel

^fe^ Gastrular ^^^^

Yolk

Dorsal blasto-

pore lip

Blastocoel

Dorsal blasto-

pore lip

germ ring '

^ Yolk plugYolk plug

ArchenteronMedullary plate | Notochordal cells

Archenter ^^^^^^^^^^^^.^^)rsal blasto-

pore lip

Yolk plug

Blastocoel

Archenteron

Ventral blasto-

ip

/ \

/ \

r Gastrular slit A

Completion^ bridge

\, NotochordArchenteron ^

Mesoderm

Yolk

BlastocoelYolk

Involuted

mesoderm

Gastrulation and early neurulation in the frog; (solid) ectoderm, (cellular)

endoderm and yolk, (striated) mesoderm, (circles) presumptive notochord

and mesoderm.

125


NEURAL PLATE—STAGE 13

NEURAL FOLD—STAGE 14

ROTATION—STAGE 15

NEURAL TUBE—STAGE 16

Development of the frog {Rana pipiens) from the yolk plug stage to the neurula.

EARLY ORGANOGENY 127

the general elongation of the entire embryo and its contained archen-

teron. At this stage, when the primary nervous structures are being

formed, the embryo is known as a neurula.

The medullary or neural plate extends from the dorsal lip of the

blastopore to the anterior limit of the developing embryo, where it

appears somewhat rounded in contour. The elevated neural folds

are therefore continuous around the margins of this thickened medul-

lary plate, the anterior junction of the folds being designated as the

transverse neural fold to distinguish it from the paired, extensive and

more posterior lateral neural folds. This transverse neural fold repre-

sents, then, the anterior extremity of the developing brain. The

regions of the lateral neural folds represent the posterior parts of

the brain and the spinal cord levels.

These lateral neural folds move toward each other and first makecontact at a point slightly anterior to the center of the original medul-

lary plate, a region which will be identified later as the level of the

medulla of the brain. From this initial point of contact the neural folds

come together and fuse in both an anterior and a posterior direction,

thus converting a groove into a closed canal, the medullary (neural)

tube or neurocoel. Obviously, fusion will occur last at the extremities,

the anterior one being called the anterior neuropore and the posterior

one the blastopore. At the posterior end the medullary (neural) folds

merge into the sides of the blastopore. As the folds meet they cover

over this blastopore and the enteron is therefore no longer opened

to the exterior (by way of the blastopore) but into the posterior end of

the neurocoel. The original blastopore then becomes a temporary

tube-like connection between the gut and the nervous system, knownas the neurenteric canal.

As the neural folds fuse and the neurocoel becomes constricted

off from the dorsal ectoderm, the latter becomes a continuous sheet

of cells above the mid-dorsal line. The enclosed canal, lined with

ciliated and pigmented ectoderm, is the neural canal or neurocoel

which is found as the much-reduced central canal of the spinal cord

and brain of the adult frog.

Slightly ventral to the anterior end of the closing neural folds there

appears a semicircular elevated ridge of ectoderm, the two exten-

sions of the elevation merging with the lateral limits of the transverse

neural fold. This is called the sense plate which contains the material of

the fifth and seventh cranial nerve ganglia. As the neural folds come to-


Early development of caudal structures. {Top, left) Posterior view of late

neurula. {Top, right) Enlargement of illustration at top, left. {Bottom) Sagittal

section through the posterior end; (dashes) nervous, {circles) entoblast (pre-

sumptive endoderm), {crosses) notochord, {dots) mesenchyme (presumptive

mesoderm). (Pasteels, Jean, 1943, Fermeture du blastopore, anus et intestin

caudal chez les Amphibiens Anoures, Acta Neerland. morphoL, 5:11.)

gether, this sense plate remains distinct (i.e., the sides do not becomefused as do the neural folds), but its posterior limits merge with the

outer margins of the lateral neural folds even after the fusion of the

latter structures. These sense plates will give rise to the mandibular

(first visceral) arches, lens of the eyes, nasal placodes, and oral suckers.

The ridges are formed largely from mesoderm and will give rise to

parts of the jaw apparatus. The superficial suckers are paired larval

organs that take the shape of an inverted "U." They become glandu-

lar and form a mucous secretion which the larva uses to adhere to

objects after hatching.

The anterior median level of the sense plate will shortly develop a

vertical groove, the stomodeal cleft, which separates the two mandib-

ular ridges (arches) or primordia of the right and left sides of the


[TAIL BUD SOMITES

IIND LIMB

YOLK MASS

NEURAL TUBE'


Sense plate

Optic plate

Optic vesicle

Stomodeal invaginationOral sucker

A face view of the 5 mm. frog tadpole.

to degenerate. The ventral limit of the vertical groove becomes the

ectodermal stomodeum when it breaks through to the endodermal

pharynx to form the mouth. That part of the mouth derived from the

stomodeum will therefore be lined with ectoderm. The dorsal limit

of the vertical groove becomes the hypophyseal invagination. Di-

rectly dorsal to each oral sucker, above and to either side of the

stomodeal cleft, there develop large oval evaginations (bulbous out-

Optic

protuberance

G'" onloge

Pronephric

region

Hypophyseainvagination

Stomodeal cleft

Anlagen of the head region: 5 mm. frog tadpole.


growths) which will be recognized as the external evidences of the

internally enlarging optic vesicles or opticoels.

Visceral Arches

Parallel to the posterior margins of the sense plate there develops

another thickened pair of elevations, directed forward. These are

the gill plates which merge imperceptibly with the more posterior

lateral folds of the sense plate and the closed neural folds. They

contain the ninth and tenth cranial nerve ganglia and represent the

forerunners of the gill or branchial arches. These are very important

in the development of the external gills used for aquatic respiration in

the tadpole. These gill plates acquire vertical grooves (furrows)

which separate the intermediate bulges into three prominent vertical

thickenings known as the visceral or branchial arches. The most

anterior of the grooves appears between the mandibular arch of the

sense plate and the first thickening of the gill plate and is known as

the hyoniandibular groove or furrow. This groove never really opens

through from the outside to the pharynx as a cleft. Just posterior to

this hyomandibular groove is the first gill thickening, the beginning

of the second visceral or hyoid arch which will provide the meso-

dermal structures to the tongue and operculum. The fifth groove, the

most posterior of all, is the next to develop. Then follow the third,

fourth, and sixth visceral grooves (the fifth developing later), divid-

ing the gill plate into vertical thickenings. The sixth is rudimentary

and posterior to the gill plate. The visceral grooves appear in the

sequence of I, II, III, IV, VI, V, counting from the anterior. The

intermediate thickenings between the grooves are the visceral arches

which remain rather solid, some of which will soon give rise to the

external gills. Those which give rise to gills are sometimes referred to

as branchial arches, the first of which is the third visceral arch.

The visceral arches, from the mandibular as the first, may be

numbered in sequence in a posterior direction, and may be designated

as visceral arches I to VI. However, since the third visceral arch be-

comes the first branchial arch because it is the most anterior arch to

give rise to an external gill, to name the arches "branchial" one must

begin with visceral arch III (called branchial arch I) and number

them posteriorly as branchial arches I to IV.

The term "visceral arch" is used because of the homology of

this structure with similar structures in other vertebrates, even though


external gills are formed. In the frog, visceral arches III to VI develop

external gills and they therefore can be properly called ''branchial

arches." A tabular comparison is given to clarify the distinction:

Original Structure Structure Formed

Visceral arch I Mandibular arch (jaw parts)" " II Hyoid arch (tongue and operculum)" " III Branchial arch I, first external gill

" "IV " " II, second external gill

V " " III, third external gill

" "VI " " IV, rudimentary fourth external gill

Three, sometimes four, of the vertical (visceral) furrows will open

through to the pharynx as clefts, functioning in gill respiration. They

open in the following order: visceral clefts III, IV, II, and V. The

presence and order of grooves therefore bears no relation to the

break-through order of the clefts.

Following the hyomandibular groove in position, each is numbered

in sequence as a branchial cleft, when all are developed. Branchial

cleft I, for instance, is the slit from the outside to the pharynx just

anterior to branchial arch II and immediately following the hyoman-

dibular groove. Most of the arches are named for the gills to which

they give rise. This is true except for the first (mandibular) and

the second (hyoid). These do not give rise to gills but to jaw, tongue,

and opercular parts. The confusion of the terms visceral and branchial

need not be serious if we remember that all arches are visceral and

are numbered from the anterior, while the third visceral arch is only

the first branchial—i.e., the first to have gills.

Posterior to the dorsal limits of the gill plate will be seen (at the

2.5 mm. stage) a slightly elongated swelling in the direction of the

embryonic axis. This is the surface indication of the internal enlarge-

ment of mesoderm known as the pronephros or head kidney. Moreposteriorly and slightly dorsal to this level may be seen the < -shaped

surface indications of the internal, mesodermal myotomes, or muscle

segments.

Origin of the Proctodeum and Tail

At the posterior end of the embryo in the neurula stage the

neural folds converge, as do the lateral lips of the blastopore, so that

they become confluent. The originally oval blastopore becomes a

vertical slit, due to the active merging of the two lateral neural folds.


1st visceral pouch

2nd visceral pouch

Pharynx

3rd visceral pouch

4th visceral pouch

5th visceral pouch

Mandibular arch

Hyomandibular cleft

Hyoid arch

Blood vessel

2nd visceral cleft

3rd visceral arch3rd aortic arch

3rd visceral cleft

4th visceral arch4th aortic arch

5th visceral arch

'5th aortic arch6th visceral arch

Pronephric tubule

Pronephric duct

Midgut

Hindgut

The 7 mm. frog tadpole: frontal section.

The lateral lips of the blastopore close together over the posterior end

of the neurocoel above and the posterior end of the archenteron

below. Internally this provides a temporary (about 1 hour) connection

between the central nervous system (neurocoel) and the gut cavity

(archenteron) known as the neurenteric canal or chorda-canal. Similar

temporary connections of these two major systems are seen in the


Aortic arches

Visceral pouch I

Viscerol groove I

Eustachian tube(visceral pouch I)

Visceral pouch II

Visceral groove I

Visceral pouch I

Visceral groove II

Visceral pouch IV

Visceral groove IV

Visceral pouch V

Visceral groove V

Lung bud

Oesophagus

EARLY STAGE

Branchial cleft I

Operculum

Branchial cleft II

Branchial cleft III

Branchial cleft IV

LATER STAGE

Development of the respiratory systems of the frog larvae.

development of most, if not all, of the higher vertebrates, including

man. This region of approximation of the folds is sometimes unfor-

tunately called the "primitive streak" because of certain homologies

with similarly developing structures in the chick embryo. The blasto-

pore at the posterior end of the neurocoel is now completely closed,

but in the meantime there has developed a new invaginating pit just

ventral to the blastopore, known as the ectodermal proctodeum.

Occasionally the closing of the dorsal blastopore and the opening

of the ventral proctodeum are connected by the aforementioned

"primitive streak." The proctodeum is the primordium of the anus,

and establishes a new ectodermally lined opening into the hindgut.

The extent of the proctodeal ectoderm can be determined in sagittal

sections by determining the limit of the invaginated and pigmented

ectodermal cells.

The body of the neurula stage is laterally compressed along the

dorsal surface but ventrally the belly region bulges with the large

yolk endoderm cells. The tail bud is formed by a backward growth of


Medullary plate

al fold

Notochord

Archenteron

Foregut

Stomodealinvagination

Heart mesenchyme

•er diverticulum

Somatic mesoderm

hnic mesoderm

Sagittal section of the open neural fold stage. (Redrawn and modified after

Huettner.)

Closing

blastopore

Proctodeum(anus)

Hindg

Midgut Oesophageal plug

IMotochord , INeurocoel

Hindgut

Rectum

Neurentericcanal

Cloaca

Proctodeum

Mesoderm

The 3 mm. frog tadpole: sagittal section

Rhombocoel

Tuberculum posterius

Mesocoel

Diocoel

Epiphysis

Infundibulum

Optic recess

Hypophysis

Oral evagination

PharynxThyroid

Heart

Bile duct

Gall bladder (liver diverticulum)


Formation of the tail bud.

tissue dorsal to the closed blastopore. As it is elongated it is provided

with both a dorsal and a ventral fin, the dorsal fin being developed

initially by the posterior growth of myotomes, with accompanying

blood vessels and nerves, to form the tail bud.

The process of neurulation, or the formation of the central nervous

system of the frog embryo, is too complicated to understand by means

of verbal instructions alone. By means of time-lapse photography

these developmental changes can be telescoped into a few minutes and

understood more clearly. About the time the neural folds close, there

are numerous surface evaginations and invaginations which indicate

correlating changes within the embryo. The neurula develops surface

cilia which tend to rotate the embryo within the fertilization mem-brane and its jelly (albuminous) coverings.

Internal Changes

The lining of the neural canal consists of the original pigmented

outer epidermal layer of the blastula which, by the time the canal

is formed, is ciliated, as is the entire outer ectoderm of the embryo.

The central canal of the nervous system, when it is formed, is there-

fore lined with ciliated and pigmented cells, later to be identified as

the ependymal layer. The bulk of the nervous system, however, is

derived from the inner layers of polyhedral cells from the original

nervous layer of ectoderm of the blastular roof. This properly named


Medullary plate

Neuralgroove

Notochord

Somitemesoderm

Proliferating

mesoderm

Limit of

mesoderm

Gutendoderm

Yolk

Neural fold

Neural crest

Coelom

Somaticmesoderm(epimere)

^Splanchnic

mesoderm

Lateral

plate

mesoderm(hypomere)

2.0mm embryo 2.3mm. embryo

Neural crest

Neurocoel

Ner\/e cord

Splanchnopleure

Somatopleure

Somite (segmental)

mesoderm

Coelom

Subnotochordolrod

Midgut

Somatic mesoderm

Splanchnic mesoderm

Yolk

2.6mm. embryo

Formation of the neural groove and neural tube from the neural (medullary)

plate.

"nervous ectoderm" gives rise to tlie neuroblasts of the central nerv-

ous system.

The roof and the floor of this neural tube are relatively thin, but

due to this nervous layer the lateral walls are very thick. The roof

is simply the region of the junction of the two neural folds. Some of

this nervous layer, at the level of dorsal fusion, is pinched off on either


side dorso-laterally to the neurocoel as the neural crests. The neural

crests are actually part of the original ectodermal neural folds which

do not form an integral part of the neural tube. The paired neural

crests extend the full length of the central nervous system, lie dorso-

lateral to that system, and will give rise to various ganglia of the

central and sympathetic nervous systems and to chromatophores. Cov-

ering the central nervous system dorsally is the reconstituted ectoderm,

derived by the fusion of ectoderm lateral to each of the neural folds

which are brought together at the time of closure of the neurocoel,

along the mid-dorsal line.

The continuous and paired neural crests, lodged between the

neural folds and the overlying dorsal ectoderm, become metamerically

subdivided by the developing somites. These crests are therefore

neural from the beginning, but are extra-neural in position in that

they are left outside the axial nervous system. These crests retain

cellular connections with the dorso-lateral wall of the developing

spinal cord and will give rise chiefly to the dorsal root ganglia of the

spinal nerves. At the level of the brain they give rise to the ganglia and

to the fifth and seventh to tenth roots inclusive. They may also give

rise to the visceral and cranial cartilages. At the body level they give

rise not only to the paired spinal ganglia but also to the sympathetic

nervous system, to the chromatophores of the body, and to the medulla

of the adrenal gland.

The medullary plate at its anterior extremity is the last region of

the central nervous system to be closed off from the exterior. The

opening from the presumptive brain region to the exterior is the

anterior neuropore, which has a homologue in the development of all

vertebrates. Due to the original spherical condition of the gastrula this

anterior region of the central nervous system curves ventrally at about

the level of the future midbrain, and this ventral curvature of the brain

persists and is characteristic of all vertebrates. The notochord, which

functions as an axial skeleton for the embryo, is ventral to the nerv-

ous system, and terminates just at this ventral curvature of the brain

(i.e., at the cranial flexure). The anterior neuropore is then found

at the anterior extremity of the embryo in the sagittal plane directly

in line with the terminated notochord, but in the roof of the brain.

Occasionally a sagittal section of a 2 mm. embryo will show a knot of

cells in this region which represents the puckered closure of the

anterior neuropore.

CHAPTER NINE

A Survey or tire Major Developinental

CJranês in tlie Early Embryo

Primary Divisions of the Brain Origin of the Somites

The Enteron or Gut Cavity Origin of the Excretory SystemThe Axial Skeleton Origin of the Mesodermal Epithelium,

The Mesoderm and Its Derivatives Coelom, and Its Derivatives

Origin of the Arches Origin of the Heart

Primary Divisions of the Brain.

The anterior vesicular expansion of the central nervous system

becomes constricted at certain levels and the walls begin to differen-

tiate and may be used as identifying landmarks of the embryonic

brain. It has just been stated that the brain floor bends ventrally

(ventral or cranial flexure) around the tip end of the notochord. This

flexure remains as a permanent feature of the brain. The brain floor

at this region is known as the tuberculum posterius (posterior tu-

bercle), and is in line with the notochord and the anterior neuropore,

and will give rise to the floor of the mesencephalon or midbrain.

Slightly anterior and dorsal to the tuberculum posterius the roof of

the brain acquires a considerable thickening for a limited distance.

This is known as the dorsal thickening and will be identified as the

roof of the mesencephalon, later to give rise to the optic lobes. Theprimary brain very rapidly develops thinnings and thickenings of its

walls, invaginations, and evaginations, all of which are part of its

differentiation.

It is now possible to de-limit the three primary parts of the embry-

onic brain. These divisions are present in all vertebrate embryos

of a comparable stage of development. They are the prosencephalon,

mesencephalon, and rhombencephalon.

Prosencephalon. This is the primary forebrain, consisting of all

parts of the brain anterior to a line drawn from the tuberculum pos-

terius to the anterior limit of the dorsal thickening, largely anterior

and ventral to the notochord. This portion of the brain develops al-

139

140 DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO

Optic vesicle n^ *5,^ î^ ^*>!>^ Optic stalk

field

Stage 14 Stage 15 Stage 16 Stage 17 Stage

Pigmentedlayer '"'^^ ''<^55^^.- Retina Pigmented layer

Retina

;\'j- Lens

- ^^^ Optic nerveOptic stalk

Stage 19 Stage 20 Stage 22 Stage 25

Development of the eye of the frog.

most immediately, giving rise to paired evaginations known as the

optic vesicles, whose walls will give rise to the various ectodermal

parts of the eye, exclusive of the lens and cornea.

Mesencephalon. This is the primary midbrain, or that portion

of the primary brain bounded anteriorly and posteriorly by the limits

of the dorsal thickening and lines drawn from these limits to the tuber-

culum posterius. It is located antero-dorsally to the tip of the noto-

chord.

Rhombencephalon. This is the primary hindbrain, or that portion

of the primary brain from the posterior limit of the mesencephalon to

the spinal cord which, at this stage, is not clearly separated from the

brain. The rhombencephalon lies entirely dorsal to the notochord,

and in the frog it is never clearly divided further, as it is in higher

forms.

Derivatives of Forebrain. At this stage of development the fore-

brain alone has distinguishing derivatives. Ventral to the notochord

there develops a vesicular outpocketing of the floor of the forebrain

known as the infundibulum. Its cells will contribute later to the forma-

tion of the pituitary gland, in conjunction with a cluster of pigmented

ectodermal cells seen between the infundibulum and the roof of the

pharynx. This cluster of cells is known as the epithelial hypophysis.

DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO 141

It is believed that the more anterior infundibulum will form the

pars nervosa or posterior pituitary gland. The more posterior hypoph-

ysis can be identified as contributing to the anterior pituitary gland

(pars distalis, pars intermedia, and pars tuberalis of mammals). At

the most ventral aspect of the forebrain, in the floor, is a pronounced

Infundibulum

Epiphysis

Hypophysis

Thyroid

Photograph of endocrine anlagen at the 5 mm. stage

of the frog tadpole.

thickening known as the optic chiasma, anterior to which is a de-

pression associated with the lateral optic vesicles. The depression is

the optic recess (recessus opticus) which is connected with the optic

stalk. Anterior to this, also in the floor, is a second thickening knownas the torus transversus which becomes the seat of the anterior and

other commissures. This structure lies within the more extensive

lamina terminalis, which represents the fused anterior neural folds

Tail fin Tuberculum posterius

Sub-notochordal rod ..„„.„„„. \ ^fesencephalon

Epiphysis

Neurocoe

Proctodeum (anus)

Yolk

Mesoderm

Hypophysis

. , Optic recessid \

Heart Optic chiosmo

Pericardial cavity

Liver diverticulum

Reconstruction of the 5 mm. tadpole in sagittal section.


around the temporary anterior neuropore. More anteriorly, but in

the roof of the forebrain, and sHghtly dorsal to a line drawn continu-

ously from the notochord, one finds an evagination of the roof known

as the epiphysis. This is the forerunner of the pineal body or gland.

Anterior to the epiphysis the roof of the brain eventually becomes non-

nervous, vascular, and folded as the anterior choroid plexus. Later

the habenular ganglia and commissure develop between the epiphysis

and choroid plexus. The only sense organs to develop by this stage

(2.5 mm. length) are associated with the forebrain, a further indica-

tion of cephalization within the central nervous system. They are the

optic vesicles, paired primordia of the eye, which begin to develop as

paired lateral vesicular evaginations of the forebrain even before this

portion of the brain is closed over dorsally. The presence of these

vesicles was described previously as being dorso-lateral to the sense

plate, in a facial view of the embryo. The connections of these vesicles

with the forebrain, at the point of the optic recess, become partially

constricted off into tubes known as the optic stalks.

At this stage the paired olfactory sense organs appear only as

olfactory placodes or button-like thickenings of the pigmented surface

ectoderm, each slightly ventral and mesial to the corresponding optic

protuberances. The auditory placodes may be seen in slightly older

stages as similar thickenings of the nervous ectoderm dorso-lateral

to the level of the rhombencephalon (hindbrain).

The Enteron or Gut Cavity.

The anterior limit of the original archenteron expands both ven-

trally and laterally beneath the notochord and the infundibulum of

the brain. This expanded cavity will give rise to the foregut and all

of its derivatives. The midgut, at this stage, is simply the tubular

archenteron dorsal to the mass of yolk endoderm. The hindgut is that

portion of the archenteron found in the vicinity of the temporary

neurenteric canal.

The foregut has a prominent median antero-ventral evagination

of its endoderm known as the oral evagination. This will make con-

tact with the head ectoderm just postero-ventral to the level of the

hypophysis. There is an opposed stomodeal invagination of head

ectoderm which will meet this endodermal evagination, later to

break through as the mouth. When these two germ layers make con-

tact, prior to the rupture, they constitute the oral plate.

DEVELOPMENTAL CHANGES IN THE EARLY EMBRYO

Neurocoel Hindgut Midgut Notochord

Foregut

143

Rectum

Proctodeum

Optic vesicle

Head mesenchyme

Oral evagination

Heart mesenchyme

Liver diverticulum

Three-dimensional representation of the late neurula stage of the frog, Ranapipiens. (Redrawn and modified after Huettner.)

The large cavity of the foregut is the pharynx, which expands

laterally to form endodermally lined visceral pouches on either side

of the developing vertical arches. These pouches are therefore verti-

cally elongated endodermally lined sacs, at all times continuous with

the pharynx. The most anterior pouch is called the hyomandibular

because it comes to lie between the mandibular and the hyoid arches.

Following this will be the first, second, and third branchial (gill)

pouches, otherwise known as the second, third, and fourth visceral

pouches. These all expand outwardly toward the opposed invagina-

tions of the ectoderm which form external visceral grooves.

A medio-ventral and posteriorly directed pocket develops from

the foregut, extending beneath the yolk a short distance. This is

the liver diverticulum, the forerunner of the bile duct, the gallbladder,

and the liver. This is the extent of gut development by the 2.5 mm.stage.

The Axial Skeleton.

The notochord was derived from cells indistinguishable from the

mesoderm at the region of the dorsal lip. These cells very quickly

expand and become vacuolated, and take on the appearance which

they will manifest throughout development until they are displaced

by bone of the vertebral column. Even the intercellular material of

the notochord becomes vacuolated. The entire group of cells be-

comes enclosed in an outer elastic sheath and an inner fibrous sheath,

both of which are classified as connective tissue.


Auditory vesicleEpimeric mesoderm

Notochord

(splanchnic

Three-dimensional representation of the tail bud stage of the frog embryo,

Rana pipiens. (Redrawn and modified after Huettner.)

The Mesoderm and Its Derivatives.

Embryonic, loosely dispersed presumptive mesoderm is known as

mesenchyme. At the end of gastrulation most of the frog meso-

derm is in the form of sheets of such loose cells extending in all di-

rections from the lips of the blastopore. However, the most anterior

mesoderm, that found in the head and pharyngeal regions, is in the

form of mesenchyme.

Origin of the Arches. Lateral to the pharynx are developed

vertical concentrations of mesoderm known as the arches. Subse-

quently most of these will contain blood vessels and nerves but at this

stage they are merely condensations of mesenchyme. The most ante-

rior arch is anterior to the first endodermal pouch and ectodermal

groove, and is known as the mandibular arch associated with the de-

velopment of the jaw muscles. This was first seen ventral to the

sense plate on either side of the stomodeal and hypophyseal cleft.

Posterior to this mandibular arch is a parallel hyoid arch, and between

these arches is developed the rudimentary hyomandibular groove

(ectodermal) and pouch (endodermal) which never break through to

form the cleft. Posterior to the hyoid arch is the first endodermal

branchial pouch followed by the first mesodermal branchial arch;

the second branchial pouch followed by the second branchial arch;

and so on as the more posterior derivatives develop. The second arch

at this stage may not as yet have its mesenchyme clearly marked

off from the succeeding arches.


Origin of the Somites. The mesoderm posterior to the pharynx

and lateral to the notochord (as seen in transverse section) taices

an inverted horseshoe shape around the archenteron and the yolk.

The uppermost level of this mesoderm, lateral to the nerve and noto-

Stomodeal clert

Pericardium

Myocardium

Endocardium

Thyroid gland

Mesenchyme

Bulbus arteriosus

Pericardial cavity

Atrium

Sinus venosus

Right vitelline vein

Liver diverticulum

Coelomic spaces

Yolk

Splanchnic mesoderm

Somatic mesoderm

rProctodeum

Frontal ( horizontal) section of the 7 mm.frog larva at the level of the developing heart.

(Redrawn and modified after Huettner.)

chord, is termed the segmental or vertebral plate and the more ventral

portion of the same sheet of mesoderm is called the lateral plate meso-

derm. This lateral plate extends continuously in a ventral direction

and surrounds the yolk, just within the belly ectoderm. Shortly, all

of this mesoderm will be separated off into a dorsal somite mass


Craniol nerve V

Notochord

Anterior cardinal vein

Olfactory pit

Diocoel

Pigmented layer

Retina

Lens

- Cranial nerve VCranial nerve VII

Otic vesicle

Cranial nerve IX

Cranial nerve XLateral line nerve

^— Somite

Neurocoel

Spinal nerve

The 7 mm. frog tadpole: frontal section.

(epimere), an intermediate cell mass (mesomere), and a lateral plate

(hypomere).

The epimere (segmental or vertebral plates) posterior to the phar-

ynx becomes divided into sections known as the metameric somites.

These are developed in sequence from the anterior to the posterior so

that at any time in the development of an early embryo the anterior

Mesenchyme

Dorsal aorta

Pronephric duct

Neural crest

DermatomeMyotome

Lateral line nerve

SclerotomeNeurocoel

Notoctiord

Post-cardinal vein

Sub-notochordal rod

CoelomSomatic mesodermSplanchnic mesoderm

Gut

Yolk

Vitelline veins

The 7 mm. frog tadpole: transverse sections. Through the mid-body level.

Ectodermepithelia

neura

Sub-notochordal rod

Nerve cord I

Neurocoel (spinal canal)

Notochord

Somite

Nephrotonne(mesomere)

Hypomere(lateral platemesoderm)

Early organogeny. The 5 mm. frog tadpole at mid-body level. Photograph

of cross section.

147


somites show the greatest differentiation. About four pairs will be

seen in the 2,5 mm. embryo. The somites sever their connections with

the intermediate cell mass and the more ventral lateral plate meso-

derm. They become blocks of cells within each of which there de-

(Mandibulor) Visceral arch I

(Hyoid) Visceral orch II

Visceral arch II

External gill I

Visceral arch IV

External gill II

Viscerol arch V

Oesophagus

Midgut

Hindgut

Stomodeal cleft

Olfactory pit

Olfactory placode

Hypophysis

Internal carotid artery

Aortic arches

Pharynx

Pronephric tubules

Splanchnic mesoderni

Somatic mesoderm

Yolk endoderm

Pronephric duct

Frontal (horizontal) section of the 7 mm.frog larva at the level of the pharynx. ( Redrawn

and modified from Huettner.)

velops a cavity, the myocoel. This myocoel is eccentric in position,

appearing displaced toward the lateral margin of the somite. As a

result of this, the outer layer of somite cells is the thinner. It is known

as the dermatome, or cutis plate, having to do with the derivatives

of the dermis and of the appendage musculature. The inner layer of


somite cells is thicker and will give rise to the skeletal muscles of

the back and body. This portion is known as the myotome. A few

NotochordNeurocoel / Dorsal aorta

Aortic orches

Internal corotidartery

Venous circulation

in liver Liver diverticulum

Externalcarotid artery

Bulbus arteriosus

Ventricle

Atrium

Sinus venosus


Visceral orches I and II

Sections of

external gills

Liver diverticulum

The earliest complete, closed blood vascular system

of the frog embryo (found at the 4 mm. stage).

Schematized drawing.

scattered cells may be seen between the myotome and the notochord,

proliferating off from the somite. These are mesenchymal cells knownas the sclerotome. They will give

rise to the vertebral skeleton.

Origin of the Excretory Sys-

tem. The mesomere and hypomere

are still connected. The lateral

plate (i.e., mesomere), separate

from the epimere (somite), nowdevelops along its dorsal border a

continuous, antero-posterior band

of mesoderm known as the nephro-

tome. This will give rise to parts

of the larval and the adult ex-

cretory systems. The nephrotomal

band enlarges in a lateral direction

but maintains cellular connections

with the remaining dorsal limit of

the lateral plate mesoderm. The

influence of the segmentation of

the vertebral plate extends to the

separated nephrotomal band, dividing it also into a series of meta-

meric nephrotomes. This nephrotomal segmentation is transitory in

THE 7 MM.

FROG TADPOLE

Frontal section through the level of

the heart of the 7 mm. tadpole.


. Diocoel

-DiencephalonInfundibulum'Anterior cardinal vein

Pigmented layer of eye

'Optic cup

Anterior tip of notochord

Rhombocoel

Rhombencephalon


Notochord

Cranial nerves VII

ond VIII

External gill

The 7 mm. frog tadpole: transverse sections. {Top) Through the level of the

thyroid gland. {Bottom) Through the level of the heart.

the frog, but persists in the embryos of some vertebrates. At about

the level of the second to the fourth somites, the center of each nephro-

tome becomes evacuated to develop a nephrocoel. This is the very

beginning of the embryonic head kidney or pronephros. The effect of

the expansion of the intermediate mass of mesoderm, due to the de-

velopment of the nephrocoel, was seen on the surface of the earlier

embryo, lying just dorsal and posterior to the gill plates.

Origin of the Mesodermal Epithelium, Coelom, and Its

Derivatives. In the more ventrally placed hypomere (lateral plate

mesoderm) we find a continuous split which separates the mesoderm

into an outer, parietal or somatic layer and an inner, visceral or

splanchnic layer of mesoderm. The outer somatic mesoderm, in con-

junction with the adjacent body ectoderm, is called the somatopleure,

and gives rise to the skin with its blood and connective tissue. The

inner splanchnic mesoderm, in conjunction with the gut endoderm.


with which it later becomes intimately associated, is called the splanch-

nopleure and gives rise to the lining epithelium, muscles, and blood

vessels of the entire mid- and hindgut.

In between these two sheets of lateral plate mesoderm the cavity

is known as the primary body cavity or coelom. Eventually the

coelomic slit becomes continuous ventrally, from one side of the

embryo to the other, forming a single visceral or coelomic cavity.

Dorsally the junction of the lateral plates is interrupted by the noto-

chord and the sub-notochordal rod. This latter structure, also knownas the hypochordal rod, is a small rod of pigmented cells between

the gut roof and the notochord. Presumably this represents a vestige

of the connection between the two at the time of their simultaneous

origin at the vicinity of the dorsal lip of the blastopore. It appears first

at the 2.5 mm. stage.

Origin of the Heart. The lateral plate mesoderm extends into

the head, ventral to the pharynx, as mesenchyme. This mesenchymebecomes organized into sheets, coextensive with the more posterior

lateral plate sheets of somatic and splanchnic mesoderm. These will

give rise to parts of the heart. As the coelomic split occurs at the

body level, there is an extension of this split into the forming heart

mesoderm, which will give rise to the pericardial cavity. The outer

Myocardium

Pericardial cavity

Dorsal mesocardium

Pericardial cavity

Myocardium

Endocardium

Pericardium

Myocardium

\ EndocardiumPericardium

Aortic arch

Blood corpuscles

Pericardium

Pericardial cavity

Endocardium

Myocardium

Development of the heart of the frog embryo.


Rhombocoel

Rhombencephalon

Endolymphatic duct

Otic vesicle

Branchiol vessel

Bronchial (gill) cleff

Opercular (gill) chamber

Neurocoe

Nerve cord

Notochor

Pronephric tubule

Coelon^

Bulbus arteriosus Ventricle2

Nerve cord

Dorsal aortaNotochord

Dorsal oorto-y

ForegutMidgut-

Somotopleure-

Splanchnopleure-

Lateral line nerve

Aortic arch

Pronephric duct

dgut

Coelom

Proctodeum

Representative transverse sections of an 8 mm. frog larva.


layer of mesoderm, corresponding to the body somatic layer, will

become the pericardial membrane. The inner layer of mesoderm,

corresponding to the body splanchnic layer, will become the myo-

cardium or heart muscle. As in the body region, the mesoderm from

the two sides grows together ventrally to fuse below the foregut.

Both the coelom and the continuous and related pericardial cavity

arise as bilateral cavities only to fuse and form single cavities

around particular organs.

The endocardium or lining of the heart arises from scattered

cells of mesodermal origin found beneath the pharynx. These cells

become organized into a sheet of epithelium as they are enclosed

by the bilateral folds of myocardium as they come together.

CHAPTER TEN

A Survey or tlie Later Embrvo or Larva

External Features

Metamorphosis

External Features.

The external changes in shape of the frog embryo and early larva

are continuous. The body is elongated and a posteriorly directed tail

bud and tail develop, just dorsal to the original position of the blasto-

pore. This tail carries with it an extension of the notochord, myo-

tomes, and blood vessels as well as the pigmented epidermis of the

body. It shows contractions of the < -shaped muscle blocks even

before the time of hatching.

The previously described surface changes are further accentuated.

The yolk mass accounts for the ventral bulge in the belly region.

Anteriorly the pronephric and gill bulges are more prominent. The

olfactory pits appear in the original placodes, dorsal and slightly

lateral to the stomodeum. All four branchial grooves are developed

and the rudiments of the external gills are beginning to grow from

the upper levels of the first and second branchial arches.

The embryo hatches when it measures about 6 mm. in total length.

Since no food is ingested during this period, the interval between

the fertilization of the egg and hatching depends almost entirely upon

the temperature of the environment during development. The hatch-

ing process probably is accomplished by the aid of temporary glands

in the sucker region. These glands presumably elaborate an enzyme

which aids in digesting away the surrounding jelly coverings, allowing

the larva to escape. The jelly itself is not a food for the larvae, or

tadpoles, even though after hatching they often are seen attached

temporarily to the empty jelly capsules.

The pre-hatching embryos show constant swimming movements

due to the presence of surface cilia. At this stage, and immediately

after hatching, there is also considerable muscular movement and

the entire larva (tadpole) may show occasional coil and S-shaped

body contractions which are entirely muscular.

155

156 A SURVEY OF THE LATER EMBRYO OR LARVA

.Olfactory pit.

Stomodeal cleft

External gills,

M Jm:... aafe:-.

Development of the external gills of Raiia pipiens.

Adaptations for respiration become a more and more important

biological function as the embryo develops organ systems. Thus the

third and fourth visceral arches develop finger-like external gills and

the fifth visceral arch gives rise to a rudimentary pair of such gills.

The mouth opens through to the gills and a constant current of water

is taken into the mouth and pharynx. This water passes out through

the second and third branchial clefts (otherwise designated as the

fourth and fifth visceral clefts) and over the associated external gills.

In this way these thin-walled, ectodermally covered, and highly vas-

cular gills are able to exchange CO.j and Oo. Subsequently, when the

tadpole develops a set of internal gills, the hyoid arch gives rise to a

posteriorly directed flap-like membrane which covers the degenerat-

Tadpole with external gills.

A SURVEY OF THE LATER EMBRYO OR LARVA 157

ing external gills. This is called the operculum. On the left side of

the head the operculum remains open at its posterior margin, to allow

the egress of water. This opening is known as the spiracle. The oper-

cular flaps from the two sides fuse ventrally to envelop the gill or

opercular chamber within. Surrounding the mouth are a pair of

horny jaws and lips, covered by horny rasping papillae. These are

derived from the corneum and consist of rows of tooth-like horny

denticles which are replaced frequently. Cornification begins at the

1 1 mm. stage.

Feeding becomes important as the yolk is being consumed more

rapidly. The tadpole begins to use its oral accessories to obtain food,

which is almost exclusively vegetation, until after metamorphosis.

The intestine, developed from the midgut, is a long, thin, coiled tube

having the appearance, through the thin abdominal wall, of a watch-

spring. If stretched out straight, this larval gut often measures about

nine times the length of the body of the tadpole.

Metamorphosis.

Under normal conditions of temperature (i.e., 20°-25° C.) and

food supply, the tadpole of Rami pipiens will reach metamorphosis

in 75 to 90 days. This period can be extended by keeping the

larvae in an environment cooler than normal or it may be shortened

by keeping them warmer and feeding them thyroid hormone or dilute

iodine which tends to accelerate the changes attending metamorpho-

sis.

There are four major areas of change during metamorphosis.

First, the respiratory system, which has already gone through an ex-

ternal and an internal gill phase, now changes to a lung type of res-

piration. This develops concurrently with the change from an aquatic

to a terrestrial environment or habitat, characteristic of amphibia.

Second, the horny jaws are lost, the mouth widens, and the gut

shortens to about two or three times the length of the body. There

are parallel changes in the histology of the gut to take care of the

change in diet. Third, the two pairs of legs develop and the tail is

lost by regression. The hind legs appear some time before metamor-

phosis and the forelimbs are pushed through the opercular membranejust before emergence of the tadpole from the water. Fourth, certain

endocrine glands function actively and the definitive gonads appear.

There are also those changes v/hich are necessarv in the transforma-

158 A SURVEY OF THE LATER EMBRYO OR LARVA

Presumptive medulla

Mesencephalon

Mesocoel

-Presumptive cerebellum

Auditory vesicle

Spinal ganglia SomitesNeurocoel

Laterol line organ ^

Cranial nerves

franiol nerve Vlil

\Epiphysis

' Diocoel

\ ' "Head mesenctiyme

ndolymphatic duct

ly VIII VII y,

Laterol line nerve '--•»»«»—

Tail Somites ^.

-'

"-^-jg^ '^ -.^ .". ..^. .. ; :. -. .. -^. _, r. . *

'.-

Proneptiric ductsOptic cup

Auditory vesicle


Notoctiord

Optic cup

Cranial nerve V

Mesenchyn

Yoii^Left coBlom Liver diverticulum

Hypoptiysis

Hindgul

•=^-'* '^z'^^^-^*^™..^-'' Pharynx

ExternolgillBranchial artery

Tail_>

Sinus venosus

Liver diverticulum I External gills

Hindqut ^- y ..j^sa^V"

Yolk

Right vitelline vem *** Bulbus arteriosus

Coelomic spaces ^^^^Swft?^**^ y_Jf^:^^ Visceral arches' ««r a. .11,... _.»

Vitelline vein

'folj'ij——— , A Sinus venosusProctodeum -..tS^^^^^^?V< ^'''^'?a>.__ .Thyroid gland

Mandibular arch

Heart

Pericordiol cavity

The 7 mm. frog larva in serial frontal sections.

A SURVEY OF THE LATER EMBRYO OR LARVA 159

tion of the tadpole into the frog. The larval skin is shed, along with

the horny jaws. The mouth becomes a large horizontal slit instead

of a simple oval opening. The gill clefts are all closed.

There is thus developed a frog in miniature, once metamorphosis

is achieved and the embryonic stages of development are passed. It

will be necessary now to return to the earlier developmental stages

and treat the various derivatives of the three primary germ layers.

The student is cautioned not to lose sight of the fact that the embryo

is a composite whole, even though we are discussing the development

of one or another of the various systems. There is functional inte-

gration which must be implied, while we are studying an isolated

system in detail.

CHAPTER ELEVEN

Tlie Germ Layer Derivatives

The Ectoderm and Its Derivatives OculomotorThe Brain Trochlear

The Prosencephalon TrigeminalThe Mesencephalon (Midbrain) AbducensThe Rhombencephalon (Hind- Facial

brain) AuditoryThe Spinal Cord GlossopharyngealThe Peripheral Nervous System ' VagusThe Organs of Special Sense The Spinal NervesThe Eye The Neural Crest Derivatives

The Ear The Sympathetic Nervous SystemThe Olfactory Organs The Adrenal MedullaThe Lateral Line Organs The Stomodeum and Proctodeum

The Cranial NervesOlfactory

Optic

The Ectoderm and Its Derivatives

The Brain

The primary embryonic brain of the frog has three main sub-

divisions. The most anterior of these, the prosencephalon, alone be-

comes further subdivided into two regions, the telencephalon and the

diencephalon. In the higher vertebrates but not the frog, the rhom-

bencephalon (hindbrain) is also subdivided. The adult frog brain then

has four major divisions, arbitrarily determined, for they merge into

one another structurally and functionally. Each of these divisions

has a specific set of characteristics and derivatives, which will be de-

scribed in sequence from the most anterior to and including the spinal

cord.

The Prosencephalon.

The Telencephalon (Secondary Forebrain). The most ante-

rior division of the forebrain is the telencephalon with its original

cavity, the telocoel. One may draw a line in a sagittal plane from a

point just anterior to the epiphysis and extending through the brain

cavity to the oosterior wall of the thickened torus transversus. This

161

162 THE GERM LAYER DERIVATIVES

RhombocoelMesocoe!

Mesencephalon

Epiphysis

Neurocoel

Notochord

Tuberculunn posterius

Diocoel

Infundibulum

Optic chiasmaOptic recess

Torus transversus

Lamina terminahs

Pharynx

Hypophysis

Oral evagmation

Early organogeny of the frog tadpole, showing the primary brain vesicles in

sagittal section.

imaginary line separates the anterior telencephalon from the moreposterior diencephalon of the forebrain. The telencephalon includes

the fused anterior neural folds which comprise the lamina terminalis.

This lamina will form a partition which will separate the paired

cerebral hemispheres by a longitudinal groove.

The telencephalon is the embryonic cerebrum. Its cavity expands

laterally to give rise to the right (first) and left (second) lateral

ventricles and the surrounding thick-walled cerebral hemispheres, at

about the 12 mm. stage. These ventricles are laterally compressed.

In the frog the cerebral hemispheres are first differentiated at the 7

mm. stage but never become very large. The two telencephalic vesicles

are partially constricted off from each other but remain connected

by way of the tubular foramen of Monro, which opens into the

common (intermediate) third ventricle, known as the ventriculus

impar. The third ventricle overlaps and connects the telocoel and the

diocoel. The nervous tissue of the cerebral hemispheres is connected

transversely by the ventral anterior commissure, which is the original

torus transversus, and the anterior pallial commissure found postero-

mesially. The cerebral hemispheres become greatly thickened with

consequent enlargement and there is a parallel reduction in the size

of the contained ventricles. Antero-ventrally each grows out toward

the anteriorly placed olfactory placodes to establish connections

known as the olfactory lobes and nerves. The olfactory lobes arise as

a pair but subsequently become fused mesially. These are rather

THE ECTODERM AND ITS DERIVATIVES 163

prominent brain structures in the adult frog. They seem to originate

together and become separated only at the point of origin of the

olfactory nerves. However, each originates from the telencephalon on

its side of the brain.

Dorsal to the anterior pallial commissure is the single median

anterior choroid plexus which develops as a thin and highly vascular

invagination from the roof just at the junction of the telencephalon

Posterior Gerebellum

choroidplexus

Optic lobe (corpora bigemina)

Mesocoel (aqueduct of Sylvius)

Posterior commissur

Hobenularganglion

Anteriorchoroid plexus

Olfactory lobe

Optic recess

Optic chiosmo

Crura cerebri

Infundibulum

Pituitary

Mammillary recess

Posterior choroid plexus

Spinal cord / Cerebell

Optic lobe

(mesencephalon)Cerebral hemisphere

Tuberculum posterius

Mesocoel

Optic recess

Optic chiasma

nfundibulum

Hypothalamus

Anterior pituitary

Late development of the brain of the frog tadpole. (Top) Pre-metamorphic

stage. (Bottom) Adult brain, schematized, reduced in size.


and the diencephalon. It extends into the diocoel and into the two

telocoels, and is therefore tri-radiate.

The Diencephalon (Thalamencephalon or Between-brain).

The third ventricle is divided between the telencephalon and the

diencephalon, but the enlarged portion found surrounded by the dien-

cephalon is identified as the diocoel. This cavity extends antero-dorsally

beneath the epiphysis, antero-ventrally into the optic recess, postero-

ventrally into the infundibulum, and laterally into the relatively large

optic vesicles.

The structural derivatives of this diencephalon include the posterior

commissure, just anterior to the dorsal limit of the mesencephalon.

Anterior to this is the epiphyseal recess, and the dorso-mesial saccular

outgrowth known as the epiphysis. This continues to grow forward

and becomes separated from the brain as a small knob of cells which

remain in the adult as the brow spot. It is presumably homologous to

the pineal gland of higher vertebrates.

Anterior to the epiphysis, in the roof of the diencephalon and be-

tween it and the anterior choroid plexus, are the habenular ganglion

and commissure. In front of this there later develops a dorsal out-

growth know as the paraphysis. In the floor just posterior to the optic

recess is the very thick optic chiasma which will contain crossed

fibers from the two optic stalks. Posterior to this chiasma is a thin-

walled pocket or trough projecting beneath the anterior tip end of the

notochord, known as the infundibulum. The cells of the infundibulum

Rhombencephalon

Auditory vesicle

Posterior cardnal vein

Vitell ne arteries

Nerve VII

Nerve V

Mesencepholon

Epiphysis

Anterior card

'

no! vein

Optic cup

Lens

Caudal vein

Coudol artery

Cloaca

(Proctodeunn) Anus


Pronephric due'

Sinus venosus

Olfactory pit

-Internal carotid

artery

Hypophysis

Oral evagmation

Pharynx

Sucker

Thyroid

Aortic arch

Reconstruction of the 7 mm. frog larva showing the major organ systems from

the right side. (Redrawn and modified after Huettncr.)


will combine with the approximated and pigmented cells of the in-

grown hypophysis to form the pituitary gland of the adult. The in-

fundibulum cells give rise to the posterior part of the pituitary gland

and retain a hollow infundibular stalk connection with the brain. The

hypophysis becomes the anterior part of the pituitary gland. During

metamorphosis the individual lobes of the pituitary gland differ, both

in gross morphology and in finer structure. The lateral and inter-

mediate lobes show poor vascularization, a simple cellular structure,

Diocoel

Infundibulum

Mesenchyme

Hypophysis

Eye / Hypophy

Infundibular sac Pharynx

Notochord

Infundibular sac Optic chiasma

Development of the pituitary gland of the frog. (A) Relationship of the

hypophysis to the infundibulum as seen in a cross section through the 5 mm.frog tadpole. (B) Same as "A" but enlarged to show the pigmented hypophyseal

cells as distinct from the gut endoderm. (C) The approximation of the hypophy-

sis and infundibulum as seen in a cross section through the 11 mm. stage.

(D) Sagittal section of the 1 1 mm. stage to show the relation of the hypophysis

and infundibulum to other parts of the brain and pharynx.


uniform staining reaction, and a rather static structural development.

The anterior lobe becomes highly vascular, consists of several types

of cells, and exhibits increasing complexity with further development.

The acidophils of the anterior pituitary gland become highly differ-

entiated and the basophils become poorly differentiated when the

thyroid gland is either poorly developed or inactive. Conversely, thy-

roid activity is correlated with hyperactivity of the basophils of the

anterior pituitary gland. This gland is formed from ectoderm, but

some is epithelial and some is brain ectoderm. Between the infundib-

ulum and the tuberculum posterius is a secondary and posteriorly

directed pocket known as the mammillary recess.

The optic vesicles begin to develop very early as lateral outgrowths

from a slightly ventral level of the diocoel. The expansion of the

diocoel provides a temporary and slight thinning of the walls of the

optic vesicles. However, as these vesicles make contact with the lateral

head ectoderm, that portion of the vesicle in contact begins to thicken

and then invaginate to form a 2-layered optic cup. The most lateral

and invaginated portion of the cup will become the retina, the mesial

layer will become the pigmented layer of the eye, and the connecting

and somewhat constricted tube the optic stalk. The nervous elements

of this optic stalk will join in the optic chiasma which contains the optic

nerve fiber tracts from the two sides. The stalk will develop around

an inverted groove (the choroid fissure) which will contain, within

the groove, accessory nerves and blood vessels which feed the retina.

The Mesencephalon (Midbrain).

This portion of the brain functions largely as a pathway of nerve

tracts between the anterior prosencephalon and the posterior rhom-

bencephalon. These tracts are found principally within the paired

ventro-lateral thickenings of the walls and floor on either side of the

tuberculum posterius. They are known as the crura cerebri.

The original dorsal thickening becomes subdivided by a median

fissure into paired dorso-lateral thickenings, at about the 1 mm. stage.

These are known as the optic lobes or corpora bigeniina. They do not

reach their full development until the time of metamorphosis. An-

terior to these lobes is the posterior commissure. From the posterior

limits of the mesencephalon and optic lobes may be seen the valvulae

cerebelli and the fourth pair of cranial nerves (trochlear) which

emerge from the dorso-lateral wall. The original cavity of the mid-


brain (inesocoel) connects the rhonibocoel (fourth ventricle) with

the third ventricle which becomes narrow and is known as the aque-

duct of Sylvius (also the iter e tertio ad quartum ventriculum).

The Rhombencephalon (Hindbrain).

This portion of the brain is clearly marked off from the mesen-

cephalon by a transverse constriction in the roof of the brain, at the

posterior limit of the dorsal thickening. It is not clearly divided farther.

There appears a slight transverse thickening in the roof of the rhom-

bencephalon which corresponds to the metencephalon of higher forms

and develops into the small cerebellum. Posterior to this the roof be-

comes broad, thin, and vascular, and folds into the rhombocoel

(fourth ventricle) as the posterior choroid plexus. The ventral and

ventro-lateral walls of the rhombencephalon are known as the medulla

oblongata from which arise the cranial nerves V to X inclusive. The

walls become thickened by fibers which form numerous pathways

from the brain and cord.

The rhombocoel or cavity of the hindbrain is known as the fourth

ventricle which communicates posteriorly with the central canal of

the spinal cord and anteriorly with the aqueduct of Sylvius of the

mesencephalon.

The Spinal Cord

The neural or central canal (neurocoel) from the beginning of its

development is laterally compressed by the thick lateral walls of the

spinal cord derived from the original neural folds. The lining cells,

coming from the dorsal epithelial ectoderm, retain both their pigment

and their cilia, and are non-nervous in function. These cells, which

continue to line the central canal of the adult, are known as the non-

nervous ependymal cells. The thick lateral walls of the spinal cord are

made up of the rapidly multiplying germinal neuroblasts (primitive

or embryonic precursors of the neurons) and the supporting small

and stellate cells known as the glia (neuroglia) cells. These cells have

the function normally ascribed to connective tissue, namely support

for the neuroblasts, but they are of ectodermal origin. The compact

glia and neuroblasts close to the inner layer of ependyma comprise

the gray matter of the cord. It is within this layer that the bulk of the

cell bodies of neurons and the commissural fibers from one side of the

cord to the other will be seen. As the cord develops further, the

168

Foregut

THE GERM LAYER DERIVATIVES

Neurocoel

RhombocoelMesocoel

Dorsal thickening

,Tuberculum posterius

Prosocoel

Epiphysis

infundibulum

Diocoel "l ProsocoelTelocoel J

Torus tronsversus

Optic recess

Optic chiasma

Epithelial hypophysis

Oral plate (stomodeal invagination)

Heart mesodermLiver diverticulum

Oesophageal plug Cerebellum

IMotochord \ Rhombocoel / lyiesence

m posterius '

physis

locoel

Telencepholon

Telocoel

Torus tronsversus

Optic recess

Hypophysis

Gall bladderLung Ventricle

Development of the brain and anterior structures of the frog tadpole. (Top)

Median sagittal section of the 7 mm. frog tadpole. {Bottom) Median sagittal

section of the 1 1 mm. frog tadpole.


anterior and posteriorly directed fibers of the various neurons are con-

centrated more laterally so that only cross sections of axons will be

seen. This peripheral area is then known as the white matter, to dis-

tinguish it from the more central and cellular gray matter of the cord.

In addition to the lateral walls of the cord, which are thick, the

dorsal wall is also thick because it consists of the fused neural folds.

Dorsal column of white

matter

Ventro-laferal columnof wtiite matter

Neurocoel (spinal canol)

Gray matter (oxons)

Ependymol cells

Dorsal root ganglion

(from neurol crest)

Afferent nerve troct

Dorsal horn ] ..

[ Neuro

Iblasts

Peripheral nerve

Spinal arterySympathetic ramus

Development of the spinal cord of the frog. (Top)

Spinal cord of the 7 mm. larva. {Bottom) Spinal cord

just before metamorphosis.

The central canal or neurocoel is therefore displaced ventrally in the

embryo and larva, but, as the cord is developed and more neuroblasts

are formed, the neurocoel assumes a more central position. Within

the enveloping connective tissue membrane which surrounds the spinal

cord may be seen several blood vessels, principally the large spinal

artery located in the mid-ventral (inverted) groove of the cord.

The Peripheral Nervous System

The development of this variegated portion of the nervous system

will be treated in the following sequence, and the description will be

continuous to the final stage of development:

1. Organs of special sense: optic, auditory, olfactory, and lateral

line organs.


2. Cranial nerves I to X.

3. Spinal nerves.

4. Other neural crest derivatives: dorsal root ganglia, sympathetic

nervous system, chromatophores, parts of the visceral and cranial

cartilage, and the medullary portion of the adrenal glands.

The Organs of Special Sense

The Eye.

The optic vesicles arise early, by the tail-bud stage, as lateral diver-

ticula from the ventro-lateral walls of the diocoel. The connection of

Ectoderm or

cornea

Opticoel

Pigmented layer

Sensory layer

rods and cones

Optic stalk"Choroid knot

Choroid fissure

Inverted groove for

optic nerve and blood vessels

Schematic diagram of the developing eye parts

of the frog.

the brain cavity with the optic vesicles becomes constricted, by the

convergence of the surrounding mesenchyme, into a tube known as

the optic stalk.

The dorso-lateral wall of each optic vesicle comes into contact with

the head ectoderm, as it expands laterally, and it is then flattened and

finally invaginated. This invagination begins ventro-laterally and is

continued obliquely mesio-dorsally. This process of invagination is

aided by a thickening of the vesicle wall between the dorsal and

ventral limits. This is the region which will later form the retina.

This thickening begins at the first point of contact (dorsal) and as a

result the connecting optic stalk is moved to a more ventral position.

THE ECTODERM AND ITS DERIVATIVES

Optic cup Lens Retina

171

Developing rods and cones Choroid fissure Pigmented layer

View into the larval eye. Rana pipiens, photograph.

The newly inverted cavity thus formed by the invagination of the

lateral wall of the optic vesicle is known as the optic cup. The rim

of the cup now exposes the pupil. The cup will invaginate further and

grow in thickness until it all but obliterates the original optic vesicle

(opticoel). The lateral (retinal) and the inner or mesial (pigmented)

layers of the optic cup are therefore brought into close proximity with

only a slit-like space remaining between, the remains of the original

opticoel. Ventrally this double-layered optic cup is connected with the

optic stalk.

The three-dimensional aspect of these changes must be understood.

Transverse, sagittal, or horizontal (frontal) sections through the eye

will appear to be essentially alike. The cup is circular with only a

ventral cut-out where the inverted optic stalk is attached. If one could

look directly into such a developing eye, after removal of the lens, the

impression would be of a horseshoe-shaped cup, the pupil of the eye,

with a groove-like opening ventral. This central cavity is the optic cup

but ventral to it is the double-layered groove of the optic stalk, known

as the choroid fissure.

The lens of the eye is formed from the superficial ectoderm by in-

vagination of the deeper or nervous layer of ectoderm opposite the


opening of the optic cup. This is brought about under the inductive

influences emanating from the dorsal rim of the optic cup by the 4

mm. stage, some time before hatching. The lens originates as a placode

or thickening in the inner or neural layer of the head ectoderm. This

placode invaginates to form a (lens) vesicle by the 5 mm. stage. This

vesicle pinches off from the head ectoderm and comes to lie within the

ring of the optic cup and is supported by a suspensory ligament by

the 6 mm. stage. The outer wall of the lens vesicle remains as cuboidal

epithelial cells and the inner wall cells become elongated as lens fibers.

The cavity is ultimately obliterated. The head ectoderm then closes

over the lens to form a new covering which, in conjunction with the

head mesenchyme, forms the double-layered cornea. This cornea is

therefore derived from ectoderm and mesoderm and becomes a trans-

parent covering of the lens by the 6 mm. stage.

After hatching, at about the 6 mm. stage, the outermost wall of the

optic cup is seen as a pigmented layer which comes into contact with

the thick inner retinal wall which thereupon begins to give rise to the

rods and cones from its outer margin. These visually sensitive elements,

the rods and cones, are fully developed by the 1 1 mm. stage. They

point away from the light source, their posteriorly directed axons

covering the exposed face of the retina. The inner margin of the retina,

i.e., that toward the optic cup, is made up of neuroblasts and their

jSbers (axons) which pass over the surface of the retina to leave the

optic cup together by way of the choroid fissure. They then pass along

the walls of the optic stalk, which acts as a guiding path to the

fibers, to reach the diencephalon of the brain as the second cranial or

the optic nerves. There is a junction and crossing of the paired optic

nerve fibers in the optic chiasma which is seen in the floor of the dien-

cephalon. The choroid fissure will eventually close around the blood

vessels and nerves which supply the optic cup. These latter nerves

have no visual function. The ventral margins of the optic cup, where

the choroid fissure originates, eventually fuse to form the choroid knot

and it is from this cluster of cells that the cells of the iris arise. Pigment

very soon disappears from the outer superficial layer of the original

optic cup.

The large cavity of the eye between the lens and the retina, des-

ignated as the optic cup, becomes filled with a viscous fluid known as

the vitreous humor. This is derived from the cells of the retinal wall

and lens and is therefore of ectodermal origin. Head mesenchyme

Development of the optic cup and lens in Siredon pisciformis. (After Rabl.)

173


ÂrCells of

vitreous

body

Dptic'stalk Diocoel Mesocoe

/V ^ \Pigmented Retina

ILens vesicle Pigmented layer

layer Lens

Lens fibers Lens epithelium

V

IDPresumptive cornea

Lens epithelium

Lens fibers \ Ins

Optic nerve

Sclerotic Sclerotic f°'°'^

coat \ cortilage/ Lamina bosalis

Outernuclear^Ti j laye

\^x—

I

Inner nuclear

.^S \ layer

r--Inner reticular

oyer

|-^ Ganglion cell

\ layer

iber

Presumptive cornea Pigmented epithelium Rods and cones

Development of the amphibian eye. (A,B,C) Progressive enlargements of

the 5 mm. stage. (D,E,F) Progressive enlargements of the 11 mm. stage.

(G,H,I) Progressive enlargements of the nietamorphic stage.


UPPER EYELID

CIUARY BODY

EPIDERMIS

SUSPENSORY LIGAMENT

OUS HUMOR

ORBITAL CAVITY

( CAPSULELENS { EPITHELIUM

( FIBERS

NICTITATING MEMBRANE

LOWER EYELID

PTIC NERVE

SCHEMATIC DIAGRAM THROUGH FROG'S EYE(REDRAWN FROM MANGOLD '31)

gives rise to the connective tissue of the choroid coat that surrounds

the pigmented layer of the eye. Outside of the choroid coat is the very

tough sclerotic coat, also mesenchymal. The nervous (sensory) parts

of the eye are therefore ectodermal in origin, but the blood vessels,

connective tissues, cartilage, and parts of the cornea are all mes-

enchymal (mesoderm).

The Ear.

The frog has no outer ear. The inner and the middle ear are de-

veloped much in the manner of all vertebrate ears, but to a less efficient

and complicated degree.

The Inner Ear. The superficially placed auditory placode de-

velops from nervous ectoderm on the side of the head at the level

of the rhombencephalon, prior to the time of hatching (at about the

2.5 mm. stage). This occurs under the inductive influences of the

medulla and archenteric roof. It moves inwardly toward the brain and


NeurocoelEndolymphatic duct

Anterior canal

Cartilage

utricle

Notochord Pharynx cartilage

Auditory apparatus of an 11 mm. frog tadpole.

invaginates to form a vesicle with but a temporary external opening at

the 3 mm. stage. This vesicle is the otocyst or auditory sac. After the

head ectoderm closes over this invaginating vesicle at the 4 mm.stage, and the walls become continuous, there develops from its dorso-

mesial wall a small evagination. This soon (at the 7 mm. stage) be-

comes tubular and is known as the endolymphatic duct. The duct on

each side grows dorsally to fuse with the duct of the other side of the

brain. It finally loses its cavity and forms a vascular membrane which

covers the rhombencephalon, having no auditory function whatever.

The duct remains as a vestige even in the adult frog, originating be-

tween the membranous labyrinth (inner ear) and the hindbrain.

At about the 11 to 1 2 mm. stage there develops a vertical fold which

divides the main cavity of the otocyst into mesial and lateral chambers.

The more dorsal and mesial portion is the utricle and the more ventral

and lateral portion is the saccule, both being joined by a small pore.

The utricle, by the 15 mm. stage, becomes further subdivided by

three folds or ridges. These develop on the inside of the utricular wall.

One is vertical and anterior (anterior semi-circular canal), one is

horizontal and lateral (horizontal semi-circular canal), and finally

there will appear a third fold which is vertical and posterior (posterior

semi-circular canal). These ridges fuse with one another to form

three loop-like tubes, each of which opens at both ends into the utric-

ular cavity with which it retains connection throughout the life of the

frog. These are known collectively as the semi-circular canals which

later become free from the utricular wall and continue to grow and


later function as organs of equilibration in the adult. The junction of

each canal with the utricle causes the local enlargement of the

utricle to form an anipuHa.

The saccule and the endolymphatic duct remain continuous. The

endolymphatic duct, however, has no auditory function but joins its

bilateral homologue to form the fused endolymphatic sacs which re-

main only as the vascular covering of the hindbrain. The saccule,

however, gives rise (at the 15 mm. stage) to two evaginations or sacs.

One of these is posterior and ventral, known as the cochlea or lagena,

and the other is slightly more dorsal (but posteriorly directed) and

is known as the pars basilaris (basilar chamber) of the ear. These

auditory structures never develop to the extent that they do in higher

vertebrates. However, in all vertebrates the lining epithelial walls of

the utricle (1), saccule (1), cochlea (3), and the various ampullae

(3) develop sensory patches (as numbered) which are connected

functionally with the eighth cranial or the auditory nerve.

The sensory (auditory and equilibratory) parts of the ear are

therefore purely ectodermal in origin. However, the entire ear is en-

Rhombencephalon

Auditoryplacode

Cranialnerve VII

perior sinus

Posterior semi-circular canal

Posterior ampulla

Pars basilaris

Lagena (cochlea)

Cranial nerve VIII

Development of the auditory apparatus of the frog. (Redrawn after Krause.)


cased in mesenchyme which gives rise to cartilage that is finally

displaced by bone. This makes up the auditory capsule which is

mesodermal. Between the capsule and the inner ear (membranous

labyrinth) is the perilymph space which is filled with the perilymph

fluid, derived from mesenchyme.

The Middle Ear. This portion of the ear grows as an endo-

dermally lined tube extending dorso-laterally from the original hyo-

mandibular pouch and developing a terminal chamber. The chamber

comes to lie between the inner ear and the head ectoderm. That por-

tion of the endoderm in contact with the head ectoderm (original hyo-

mandibular plate) becomes the tympanic membrane (ear drum),

later to be invaded by mesenchyme which forms blood vessels and

connective tissue. This membrane is supported by a cartilaginous ring

known as the annulus tympanicus (tympanic ring) which is believed

to arise from the palatoquadrate bone. These latter two structures are

presumed to cause the development of the tympanic membrane by

induction.

This endodermally lined chamber expands inwardly to make con-

tact with the inner ear capsule at the site of the foramen ovale, an

aperture in the inner ear capsule and extending through the perilymph

space. This aperture is obstructed by a cartilaginous plug known as the

operculum. This operculum later becomes ossified into bone. Acartilaginous rod arises from the roof (dorsal wall) of this cavity and

extends from the operculum of the inner ear to the tympanic mem-brane and across the middle ear cavity, and is known as the columella

or plectrum. This columella is mesodermal, arising from the inner

stapedial plate (part of the otic capsule) and a cartilage from the

palatoquadrate bar. The columella becomes partially ossified and con-

veys to the inner ear the auditory vibrations which impinge upon the

outer tympanic membrane. It is derived presumably from part of the

second or the hyoid arch.

The original connection of the pharynx and the middle ear cavity,

the dorso-lateral tubular growth from the hyomandibular pouch, re-

mains in the adult as the tubo-tympanic cavity or the Eustachian tube

and is lined with endoderm.

The Olfactory Organs.

The external olfactory placodes arise as thickenings in the neural

ectoderm of the sense plate, dorso-lateral to the stomodeum, at about


Olfactory

tube

Olfactory

epithelium

Internal nores (choona)Lateral appendix Olfactory cartilage

External and internal nares of the 11 mm. frog tadpole. (Left) External nares.

(Right) Internal nares (choana) opening into the pharynx.

Pharynx

the 2.5 mm. body length stage, long before the time of hatching. Theoverlying superficial epithelial ectoderm disappears so that the nerv-

ous layer of the placode becomes* the exposed lining of the olfactory

pit.

After hatching (6 mm. stage) a solid rod of ectodermal cells grows

ventro-laterally from the olfactory pit to become attached to the

pharynx just dorsal to the oral plate (i.e., stomodeum). By the 1 1 mm.stage this core of cells acquires a lumen which is continuous from the

external nares (olfactory pits) to the internal nares (internal choanae)

which open into the pharynx. The major part of this olfactory tube is

lined with non-sensory epithelium. It develops a dorsal and a ventral

chamber (sacs), each of which acquires glandular masses which are

known as the organs of Jacobson.

The neuroblasts of the olfactory placodes send extensions pos-

teriorly and give rise to the fibers which form the olfactory or first

cranial nerve. These grow toward the brain and are guided in their

directional development by the outgrowth of the telencephalon knownas the olfactory lobes.

The Lateral Line Organs.

The most posterior or fourth cranial placode (cranial nerve X)

sends a growth posteriorly beneath the lateral body epidermis, on either

side, beginning at about the 4 mm. stage. It grows posteriorly to the


Neural ectoderm

Superficio

ectoderm

Olfactory

pit (externa

nares)

Olfactory

placode

Olfactory

nerve (I)

Headmesenchyme

Lateral

appendix

Choanalcanal

Internal nares (choano)

Pharynx

Development of the olfactory organ of the frog. {Top, left) Sagittal section

through the olfactory placode and nerve. {Top, right) Posterior transverse

section through the choanal canal. {Bottom) Schematic reconstruction of the

embryonic olfactory organ.

tip of the tail. Along these cords arise groups of sensory cells which

grow through the epidermis to become exposed along the sides of the

body as the lateral line system. The exposed cells are ciliated, and are

therefore sensitive to vibrations in the surrounding aquatic medium.

They are protected by inner and outer sheath cells and a basement

membrane and are connected functionally with branches of the lateral

line nerve. At the head level the extensions of this system seem to be

innervated by cranial nerves VII, IX, and X, principally the latter.

This structure is no doubt a vestige of the aquatic ancestry of the

Anura, for, as in lower forms, these organs are innervated by a branch

of the vagus (X) ganglion, known as the lateral line nerve or ramus

lateralis. In the Anura this system disappears by the time of meta-

morphosis.


Vagus (X)gangli

Sensory cilia

Sensory cell of loterol

line organ

Splanchnopleure

4mnn stage- transverse

section

Lateral line nerveextending into tail Lateral line

sense organ Vagus (X) nerve and

origin of lateral line

system

Tadpole

Origin of the lateral line sense organ system in the frog larva.

The Cranial Nerves

These nerves, as in the case of practically all nerves of the embryo,

are developed in pairs. Many of them are both sensory (afferent) and

motor (efferent). Each nerve will be dealt with as a separate entity.

As the neural folds close they leave a column of cells between the

fold and the dorsal ectoderm. These are known as the cranial or

neural crests, depending upon the level under consideration. The cra-

nial crests of the brain level are different from the neural crests of the

spinal cord level in that they become divided into four large pairs

known as the cranial placodes. These consist of thickened patches of

neural ectoderm of the head, one of which, the auditory placode, has

been described already. The original cellular connection between each

cranial crest segment and the brain remains as the sheath of the nerve

fibers to be developed in that region. At the spinal cord level the seg-

ments are metameric and will be as abundant as the spinal nerves de-

veloped therefrom.

Four ganglionic masses are found in the development of the frog

cranial nerves as early as the 6 mm. stage. These include the first

or semilunar ganglion of the fifth nerve. The second is the acustico-

facialis (VII-VIII) ganglion which becomes associated with the

auditory placode. The third is the glossopharyngeal (IX) and the

fourth is the vagus (X). The third and fourth arise together as the post-

otic ganglionic masses and each becomes associated with an epi-

branchial placode.


Telencepholon

Trigeminal ganglion

Acuslico-faciolis gangli

Glossophoryngeal gangi

,Vogu5 ganglion

Medullofy p'ote

Mesencepholon

Optic lobes

Cerebellum

Cranial nerve I

E«ternol nares

Nasol tube

Lateral appendu

Epiptiysis

II

Eye

Endolymptiatic duct

OtiC capsule

Vagus (X)

Lateral line nerve (Xl

Origin and derivatives of the cranial ganglia. (A) Origin of the cranial

ganglia: frontal section. (B) Relation of the crest segments and placodes:

transverse section. (C) Relation of the sense organs to the cranial ganglia.

(D) Brain, sense organs, and cranial nerves of the 12 mm. tadpole.

Embryologically the rudiments of all of the cranial nerves are more

complex than are the rudiments of the spinal nerves. They are made

up of cell masses from the neural crests, from ectodermal patches on

the surface of the head, and from cell processes extending out from

the ventro-lateral brain neuroblasts. The spinal nerves do not have

the surface ectodermal constituent. Following is a summary of the

cranial nerves of the frog embryo:


I. Olfactory: This is sensory, arising after hatching (11 mm.)

from neuroblasts of the olfactory lobe (telencephalon) and

growing to the olfactory placode.

II. Optic: This is sensory, arising at an early stage (6 mm.) from

the neuroblasts of the retinal layer of the optic cup to grow

along the ventral wall of the optic stalk to join the lateral

wall of the diencephalon.

III. Oculomotor: This is motor, arising Just before hatching from

the ventro-lateral wall of the mesencephalon (crura cerebri)

to innervate four muscles of the eye. These are the rectus

superior and the ciliary ganglion, the rectus inferior, the

rectus medialis, and the obliquus inferior.

IV. Trochlear: This is motor and remains very small. It arises late

in development from near the roof of the mesencephalon be-

tween the optic lobes and the cerebellum just posterior to the

valvula cerebelli and the posterior commissure. It innervates

the superior oblique muscles of the eye.

V. Trigeminal: This is a mixed nerve, also known as the trifacial

ganglion, which appears as early as the 5 mm. stage and has

a complicated origin from the dorsal (superficial) ganglionic

elements of the first cranial crest segment and the ganglionic

Segments of

neural crest

Nerves derivedfrom them


portion of the first cranial placode. It is well developed by the

9 mm. stage. The level is at the anterior end of the medulla.

This is a very large and tri-radiate ganglion, including an

Ophthalmic branch from the placode whose fibers pass to the

skin and snout, and a forked Gasserian ganglion which arises

from both the crest and placode elements and gives rise to the

maxillary (upper) and mandibular (lower) branches of the

fifth nerve. The entire mixed trigeminal nerve ganglion (V)

joins the dorso-lateral wall of the medulla oblongata. Theouter or non-nervous portion of the placode disappears and

it is believed that the non-nervous portion of the crest seg-

ment may contribute some of the mesenchyme to the mandib-

ular arch. It is certainly difficult to distinguish between them.

VI. Abducens: This is a motor nerve arising late (11 mm. stage)

from the neuroblasts of the ventral side of the medulla and

innervating the lateral or external rectus and retractor bulbi

muscles of the eye.

VII. Facial: This is a mixed nerve, arising by the 5 mm. stage from

the anterior ganglionic portion of the second or acustico-

facialis (second cranial) crest segment and placode and is as-

sociated with neuroblasts coming from the medulla just poste-

rior to cranial nerve V. The motor (efferent) fibers are divided

between the hyoid and the palatine (mouth) branches, but a

good portion of the crest segment, from which these fibers

arise, is non-nervous and is presumed to give rise to mesen-

chyme of the hyoid arch.

VIII. Auditory: This is a sensory nerve arising by the 5 mm. stage

from the posterior ganglionic portion of the second (acustico-

facialis) crest ganglion, in close association with the auditory

placode, and innervates only the inner ear.

IX. Glossopharyngeal: This is a mixed ganglion, arising by the

9 mm. stage from the ganglionic portion of the third cranial

crest segment and placode. Fibers of the ninth and tenth

cranial ganglia enter the sides of the medulla together, but

peripherally the ninth or glossopharyngeal nerve supplies the

first branchial arch, the mouth, the tongue, and the pharynx.

This ganglion is incompletely separated from the vagus by

the anterior cardinal vein.

X. Vagus or pneumogastric gangHon: This is mixed and arises


by the 9 mm. stage from the fourth cranial crest segment and

placode and becomes associated with the neuroblasts from

the medulla. Peripherally the branches feed the second

and third branchial arches, the tympanum, the muscles of

the shoulders, the viscera (including larynx, oesophagus,

stomach, lungs, and heart), and the lateral line system. The

non-nervous portion of the crest segment forms mesenchyme

and the superficial non-nervous elements of the placode

then disappear.

Summary of Derivatives of the Cranial Nerves in Tabular Form

Cranial Crest Segment Cranial Nerves Derived Therefrom

( V ophthalmic branch

V mandibular branch

II VII and VIII

III IXIV X

The Spinal Nerves

The spinal nerves, aside from their component parts, differ from

the cranial nerves in that they are related to mesodermal somites rather

than visceral clefts.

The spinal nerves arise from the pair of continuous neural crests.

These are elongated strands of neural ectoderm left outside of the

dorsal neural tube, between it and the dorsal ectoderm, at the time of

closure of the neural tube. These crests become metamerically seg-

mented and are paired as a result of the development of the somites. In

frontal sections of the 7 mm. stage these crests will be seen as dorsal

root ganglia of the spinal nerves. No placodes are developed in asso-

ciation with any part of the neural crests, as were described in associa-

tion with some of the homologous cranial crests.

Each crest segment is made up of many neuroblasts which send

fibers to the dorso-lateral wall of the spinal cord to form the dorsal

root or ramus of the spinal nerve. This connects the dorsal root

ganglion (original neural crest) with the spinal cord. Also, other fibers

grow ventro-laterally from this ganglionic crest to become the afferent

tracts from the skin and other sensory organs.

Within the spinal cord, as early as the 4 mm. stage, neuroblasts de-

velop and send out bundles of efferent axons (fibers) which emerge

from the ventro-lateral wall of the spinal cord and almost immedi-


Ectoderm

neura

epithelia

Neural crest

lêr\/e cord

Neurocoel

\^ Sclerotome

Myotome

Dermatome

Mesomere

Early organogeny. The 5 mm. frog tadpole at mid-body level. Photograph of

cross section.

ately co-mingle with the afferent fibers, already described. The ventral

nerve roots or rami are entirely efferent, the dorsal nerve roots are

entirely afferent, but the nerve trunks contain, within a commonsheath, both the afferent and efferent fiber trunks. Many of these spinal

nerve trunks fork to send both types of fibers dorsally to the sense

organs and ventro-laterally to the muscles and to the limbs.

The tadpole may have a total of 40 or more pairs of spinal nerves,

but only the anterior 10 pairs remain after the degeneration of the tail

at metamorphosis. The first pair (hypoglossal) emerges from between

the first and the second vertebrae, to innervate the tongue and some

of the muscles of the hyoid arch. The second pair (brachial) emerges

from between the second and the third vertebrae and are very

large. They have connections with the first and third spinal nerves and

thus form the large brachial plexus, innervating the forelimbs and the

muscles of the back. The third pair has the aforementioned connection

with the brachial but it then supplies the external oblique and trans-

verse muscles and the skin. The fourth, fifth, and sixth spinal nerves

supply the skin and muscles of the abdominal wall. The seventh,

eighth, and ninth spinal nerves together form the lumbosacral or

sciatic plexus which innervates the posterior abdominal region and


Noto- White Neural Gray Dorsal Ramuschord matter canal matter root communicans

Ventral

root

Noto- Sympa- Dorsal Ramus Muscle Muscle Noto- Caudal Neural Nerve

chord thetic aorta commu- chord artery canal cord

ganglion nicans

Development of the spinal cord and sympathetic nerves. (A) Showing the

dorsal root (from neural crest). (B) Showing the ventral root (from cord

neuroblasts). (C) Showing connection between the spinal nerve and the sympa-

thetic ganglion, known as the ramus communicans. (D) A section in the tail

region to show the relatively large notochord and reduced nerve cord.


the hind legs. The tenth, with a branch from the ninth, forms the

ischio-coccygeal plexus which sends branches to the urinary bladder,

cloaca, genital ducts, and posterior lymph hearts.

The Neural Crest Derivatives

The Sympathetic Nervous System.

The development of the sympathetic nervous system is not under-

stood clearly but it is believed also to arise from the neural crests, by

a downgrowth along the afferent trunk to the spinal nerve and thence

to the dorsal aorta and the viscera beginning at about the 6 mm. stage.

The sheath cells probably come from the neural tube itself. Just before

hatching there may be seen clusters of cells around the junction of the

dorsal and ventral nerve roots, and these clusters will give rise to the

sympathetic ganglia which become associated with the dorsal aorta

and the viscera. These ganglia are also interconnected by paired longi-

tudinal strands on either side of the spinal cord. The fibers which join

the sympathetic ganglia with the spinal nerve are known collectively as

the ramus commuiiicans.

The sympathetic system arises with the vagus (cranial nerve X),

and its connecting fibers extend through the jugular foramen of the

Efferent nerve

Sclerotome(skeleton)

Spinal cord

Dorsal root ganglion

Afferent nerve

Myotome (striated muscle)

Peripheral nerve

1Spinal nerve

Notochord

Dorsal aorta

Sympathetic ramus (communicans)

Sympathetic ganglion

Sympathetic nerve to gut

Relation of the spinal and the sympathetic nervous systems.


skull and along each side of the spinal cord to receive these con-

necting rami from the segmental spinal nerves. Both independent

sympathetic nerves and plexi are developed and are found often quite

removed from their source of origin.

The Adrenal Medulla.

The large brown cells of the inner portion of this endocrine organ

arise some time before metamorphosis by migration from the sympa-

thetic ganglia and hence are of ectodermal origin. They have lighter

staining nuclei and are scattered among the more abundant cortical

cells. These latter cells arise from the mesoderm and will be discussed

subsequently.

The Stomodeum and Proctodeum

The stomodeum and proctodeum are also of ectodermal origin, each

being represented by an invagination of ectoderm at either end of the

digestive tract. The stomodeal ectoderm meets the oral endodermal

evagination to form the oral plate which at the 6 mm. stage ruptures

through to form the mouth. Therefore, a portion of the mouth (to the

tongue) is lined with ectoderm. The proctodeum is a similar caudal

invagination of ectoderm which meets the evagination of the anal

endoderm resulting in the formation of a temporary anal plate. This

shortly ruptures. The anus is thus partially lined with ectoderm.


Summary of Embryonic Development of the 11 mm. Frog Tadpole:

Ectodermal Derivatives

Epidermis—thickened, ciliated on tail only.

Central Nervous System—differentiated into four brain vesicles and spinal

cord.

Prosencephalon (Forebrain)

Telencephalon—anterior to optic recess; thin-roofed.

Cerebral hemispheres develop around each of lateral ventricles.

Anterior choroid plexus as median dorsal invagination into telocoel

which becomes vascularized.

Olfactory (I) nerve from floor of cerebral hemisphere to mesial side

of nasal tube.

Diencephalon—from epiphysis to dorsal thickening to tuberculum

posterius.

Epiphysis as mid-dorsal evagination just posterior to choroid plexus.

Optic recess—just posterior to torus transversus or region of lamina

terminalis (anterior fusion of neural folds) as median ventral evagina-

tion.

Optic chiasma—thickened floor where optic nerve enters (posterior to

recess).

Optic (II) nerve—from chiasma to eyes, may be seen entering retina.

Infundibulum—bulbous evagination in floor of diencephalon between

optic chiasma and tuberculum posterius, ventral to notochord.

Mesencephalon—bounded by dorsal thickening and tuberculum posterius.

Optic lobes from dorsal thickening, bi-lobed, become corpora bigemina.

Cavity—mesocoel, iter, or aqueduct of Sylvius.

Oculomotor (III) nerve—mesio-lateral, from floor.

Trochlear (IV) nerve—dorso-lateral, just posterior to optic lobes and diffi-

cult to find, being small.

Rhombencephalon—from dorsal thickening to spinal cord, metencephalon and

myelencephalon not distinguishable in the frog.

Cerebellum—from thickened roof, anterior.

Posterior choroid plexus—from thin, vascular posterior roof.

Otic vesicles—found at level of cerebellum.

Cranial nerves V to X are associated with this portion of the brain.

Trigeminal (V) nerve—anterior to otocyst, large, sends branches to

mandibular and maxillary processes of first visceral arch.

Abducens (VI) nerve—from floor of rhombencephalon, anterior and

ventral to origin of trigeminal. Supplies eye muscles as do oculomotor

and trochlear.

Facial (VII) and auditory (VIII) nerve—arises as a single ganglion, mesio-

posterior to otocyst, supplying facial muscles and saccule and utricle

of ear.

Glossopharyngeal (IX) nerve—arises posterior to otocyst, sending

branches to first branchial arch. Ganglion.

THE ECTODERM AND ITS DERIVATIVES

Cranial nerve i

191

Cranial nerve II

Optic chiosmo

Pituitary

gland

Ventral Dorsal

Brain of the adult frog.

Lateral

Vagus (X) nerve—arises with IX, sending branches to second, third, and

fourth branchial arches; to lateral line organs and to viscera.

Cranial Crest Segments—visceral cartilages, parts of the cranial cartilages

from head mesenchyme originally derived from ectoderm.

Spinal Cord—from rhombencephalon into tail, with continuous cavity.

Neural (central) canal—original neurocoel, constricted centrally.

Ependymal layer—elongate, ciliated cells lining the canal.

Mantle layer—gray matter consisting of compact cell bodies, lateral.

Marginal layer—white matter, consisting of outermost axons.

Dorsal root—afferent fibers passing dorso-mesially to join dorsal root

ganglion.

Dorsal root ganglion—paired thick bundle of neurons dorso-lateral to

spinal cord, derived from neural crests.

Ventral root—efferent fibers passing ventro-laterally from spinal cord to

spinal nerve trunk.

Spinal nerve—fusion of afferent and efferent fibers associated with distal

organs (muscles, etc.).

Dorsal ramus—branch of spinal nerve to dorsal muscles.

Ventral ramus—branch of spinal nerve to ventral muscles.

Communicating ramus—fibers from spinal nerve to sympathetic ganglion.

Sympathetic ganglion—nerve cells lateral to dorsal aorta, derived from

neural crest.

Special Sense Organs—found in the head region.

Eye—well developed in 11 mm. stage, some parts mesodermal.

Retinal layer—sensory portion consisting of rods and cones (inner and

outer granular layers).

Pigmented layer—thinner layer outside of retinal layer.

Lens—arising as vesicle, now a solid ball lying in opening of optic cup.


Cerebellum

Mesencephalon

Tuberculumposterius

Epiphysis

Prosencephalon

Optic chiosma

Optic recess

Torus transversus

nfundibulum

Sinusvenosus

VentricleBulbus arteriosus Hypophysis

Sagittal section through the anterior end of the 8 mm. frog larva.

*Choroid coat—thin, vascular layer of mesenchyme immediately around

the eye.

*Sclerotic coat—loose mesenchyme investing choroid layer.

*Cornea—extension of sclerotic layer and underlying the epidermis, there-

fore both ectoderm and mesoderm.

*Eye muscles—small bundles of muscles v^hich control eye movements by

way of cranial nerves III, IV, and VI.

Ear—arising from auditory placode, then otocyst.

Utricle—mesial and dorsal, giving rise to the three semi-circular canals,

two of which have developed, i.e., anterior dorso-ventral and outer

horizontal canals.

Saccule—outer and ventral portion of original otocyst.

Endolymphatic duct—between utricle and brain, joining saccule.

Cochlea—mesial portion of saccule, pigmented and ciliated.

Nose—arising from olfactory placodes.

External nares—glandular organ of Jacobson, opened to exterior.

Internal nares—extension of tubular opening from external nares into

pharynx, called choana.

Stomodeum and Proctodeum—ectodermally lined openings into mouth and

cloaca.

"Part or all of the starred structures are derived from mesoderm.

CHAPTER TWELVE

TJae Endodermal Derivatives

The Mouth (Stomodeum, Jaws, Lips, The Thyroid GlandEtc.) The Tongue

The Foregut The LungsThe Pharynx The Liver

The Thymus Gland The Pancreas

The Carotid Glands The Oesophagus and StomachThe Parathyroid Glands The MidgutThe Ultimobranchial Bodies The Hindgut

The endoderm gives rise to all structures associated with the origi-

nal archenteron, from the mouth (ectodermal stomodeum) to the

anus (ectodermal proctodeum), and all of its derivatives. It must be

emphasized, however, that the endoderm contributes only the lining

epithelium of these structures and that (since each of them is invested

with blood, connective and nervous tissue, and often with muscle)

ectoderm and mesoderm may also be involved. It is nevertheless con-

venient to describe in sequence from the anterior to the posterior (fore-

gut, midgut, and hindgut) those structures whose linings are endo-

dermal in origin.

The Mouth (Stomodeum, Jaws, Lips, Etc.)

The lips and anterior lining of the mouth are ectodermal and the

margin between the ectoderm and endoderm can be determined in the

larva by the extent of the invaginated and pigmented stomodeal

ectoderm. This ectoderm plus the oral endoderm form the oral plate,

or oral membrane, which generally ruptures through shortly after the

time of hatching (6 mm. stage) to form the mouth opening. The

lateral margins of the mouth (stomodeum) are the original mandib-

ular ridges between which are the dorsal and the ventral (larval)

lips. These are transitory but important feeding organs of the tadpole.

The dorsal lip develops three medially placed rows of superficial teeth

which are periodically sloughed off and replaced. The ventral lip of

the larva develops four rows of somewhat more complete teeth, but

all teeth are covered with stomodeal ectoderm. A cornified ectodermal

193

194 THE ENDODERMAL DERIVATIVES

"jaw," consisting of a hardened ridge, develops at the base of both

the dorsal and the ventral lips.

By the time of metamorphosis the horny larval teeth and jaws are

lost and the mandibular arches give rise to the jaw elements of the

adult. The upper larval teeth are replaced by permanent teeth which

bear only superficial resemblance to the teeth of mammals.

The tongue, which is ultimately attached anteriorly and will be free

posteriorly, begins to develop by the time of metamorphosis from a

proliferation of cells from the endodermal floor of the pharynx. Thebulk of the tongue will, of course, be of mesodermal origin but most of

its covering and glandular elements come from the endoderm, the

anterior portion being covered with stomodeal ectoderm.

The Foregut

This anterior portion of the original archenteron expands widely in

front of the yolk mass and consists of all of the derivatives from the

stomodeum to and including the pancreas and liver. The stomodeum,

the anterior covering of the tongue, and the roof of the mouth anterior

to the internal choanae are ectodermal and only the posterior cover-

ing of the tongue and mouth, beginning at about the level of the

thyroid gland, is endodermal. This foregut portion of the archenteron

therefore assumes a major role in the early development of the frog,

where it gives rise to three successive sets of respiratory organs (i.e.,

external and internal gills, then the lungs) to most of the endocrine

organs, and to a good portion of the digestive tract.

The Pharynx.

The pharyngeal cavity is widely expanded anterior and ventral to

the yolk mass of the frog embryo. The earliest elongated and vertical

evaginations of the pharyngeal endoderm are known as the visceral

or branchial pouches. Those of the most anterior pair, which develop

between the mandibular and the hyoid arches, are known as the

hyomandibular pouches. The dorsal remnant of this first pair of

pouches gives rise to the Eustachian tube of the adult and connects

the pharynx with the middle ear chamber. The second and third pairs

of visceral pouches are known as the first and second branchial

pouches, because they give rise to gill clefts, and they develop between

the succeeding visceral arches. Eventually, six pairs of endodermal

evaginations develop in this manner, but those of the last (most

THE FOREGUT

Visceral arches

195

IV V External gills

Hyoid arch

Mandibular arch

Visceral groove

III IV V

Aortic arches

Frontal (horizontal) reconstruction of the external gill stage of the frog larva.

(Redrawn and modified after Huettner.)

posterior) pair rarely open as pouches, remaining as mere cords of

endoderm. By definition we use the term "pouch" for the endodermal

evagination, "groove" for the ectodermal invagination, and "cleft"

for the combination of ectoderm and endoderm which meet when these

grooves and pouches break through. Visceral pouches I to V inclusive

appear in this order and grow laterally toward the head ectoderm,

all but the first and fifth, meeting a corresponding groove and break-

ing through to form clefts. Occasionally a sixth visceral pouch is de-

veloped, but it is generally vestigial.

These clefts can be classified in tabular form as follows:

Pouch (Endoderm)

Arch (Mesoderm)

Cleft (Ectoderm andEndoderm)

Groove (Ectoderm)

Visceral pouch I

'Does not perforate.

Visceral arch I—mandib-

ular

(Aortic arch I)

* Visceral cleft I

(Hyomandibular—

Eustachian tube.

Ectodermal plate

—

tympanic membrane)

Visceral groove I


Pouch (EndodermArch (Mesoderm)

Cleft (Ectoderm andEndoderm)

Groove ( Ectoderm )

Visceral pouch II

(Branchial pouch I)

Visceral pouch III

(Branchial pouch II)

Visceral pouch IV(Branchial pouch III)

Visceral pouch V(Branchial pouch IV)

Visceral arch II—hyoid

(Aortic arch II)

(Opercular fold arises at

9 mm. stage, covers

gills at 10 mm. stage)

Visceral cleft II

(Branchial cleft I)

Visceral arch III

(Branchial arch I)

(Aortic arch III—ca-

rotid)

(External gill at 5 mm.stage)

Visceral cleft III

(Branchial cleft II)

Visceral arch IV(Branchial arch II)

(Aortic arch IV—sys-

temic)

(External gill II at 5

mm. stage)

Visceral cleft IV(Branchial cleft III)

Visceral arch V(Branchial arch III)

(Aortic arch V)(External gill III at 6

mm. stage)

Visceral cleft V(Branchial cleft IV)

Visceral arch VI(Branchial arch IV)

(Aortic arch VI—pul-

monary)

Visceral groove II

(Branchial groove I)

Visceral groove III

(Branchial groove II)

Visceral groove IV

(Branchial groove III)

Visceral groove V(Branchial groove IV)

The first four pairs of branchial clefts constitute channels or gill

slits from the pharynx to the exterior, and are lined with both ecto-

derm and endoderm. The first visceral (hyomandibular) cleft and the

sixth visceral cleft are rudimentary in that they rarely open through to

the outside.

THE FOREGUT 197

The finger-like and branched external gills arise as outgrowths of

the lateral wall of the third, fourth, and fifth visceral (branchial I, II,

III) arches shortly after the gill clefts are perforated. The first and

second external gills arise at the 5 mm. stage and the third at the

6 mm. stage of development. The arches carry with them the covering

ectoderm and the capillary loop of blood vessels and the nerves that

are always found within the arches themselves. The ectodermal cover-

ing is very thin so that the relatively large capillaries in each gill can

readily exchange CO._. and O.. in the surrounding aqueous medium.

These external gills constitute the respiratory organs of the larva from

about the fifth to the tenth days, and then the gills begin to atrophy.

This process of degeneration is assisted by the development of a

posterior growth from the hyoid arch, known as the operculum. This

is a membrane which covers the gills and forms, along with a similar

membrane of the other side, a ventral and ventro-lateral opercular

chamber. While the external covering of the operculum is ectodermal

it contains mesoderm. Water taken in through the mouth continues to

pass over the external gills, within the opercular chamber, but escapes

through the spiracle at the posterior margin of the left opercular fold.

In the meantime the internal gills must develop in order to take

over the respiratory functions that gradually are being relinquished

by the external gills. These internal gills develop a double row of

filaments, ventral to the branchial arches and arising doubly from the

postero-external (anterior and posterior) faces of the same first three

pairs of branchial arches. These are visceral arches III, IV, and V.

The fourth pair of branchial (sixth visceral) arches may also give rise

to reduced single internal gills from their anterior faces only. These

gills are termed internal because of their position on the arches and

also because of the fact that they are covered by a body flap, the

operculum, and are therefore truly within the body. The entire oper-

cular (branchial) chamber is lined with ectoderm, however, and this

includes the covering of all of the external and internal gills. The

internal position of this new set of gill filaments is therefore secondary.

The ventro-lateral position of the external gills, plus the develop-

ment of the operculum, tends to move them to a position beneath

rather than lateral to the pharynx, within the spacious opercular cavity

or chamber. The original endodermal branchial pouches, lateral to

the pharynx, become partially separated off by lateral projections of

the pharyngeal floor and also by longitudinal folds in the lateral aspects


Pharynx Dorsal aorta

Anterior

Cranial n

Otic copsu

Aortic arch

Remnant ofexternal gills

Operculargroove

Opercular char

Internal gills Heart

Pericordial cavity

Relation of the pharynx to the internal and external gills of the frog, transverse

section.

Spiracle (left

side only)

of the roof. This provides a lengthwise pair of pockets between the

pharynx and the latero-ventral gill chambers. From the floor of each

of these pockets develop finger-like, endodermally covered, frilled

organs known as the gill rakers. These tend to filter the water as it

passes from pharynx to gill chamber, removing any relatively large

particles of matter and retaining them within the pharynx to be carried

into the oesophagus with the food. In addition, the shelf-like projec-

tion or flap of tissue from the floor of the pharynx and also from the

lateral wall of the pharynx likewise aid in the mechanical sifting or

filtering of the water. These shelves are known as the velar plates.

During subsequent metamorphosis both the gill chamber and the

related opercular chamber become filled with rapidly multiplying cells

which ultimately become incorporated into the body wall.

The Thymus Gland.

The glandular derivatives of the various branchial pouches maybe classified as epithelioid bodies, since they are all lined with endo-

dermal epithelium. The most anterior derivatives are the paired thymus

glands which arise by the proliferation of cells from the dorsal ends

of the hyomandibular and the first branchial (second visceral)

pouches. The bulk of the gland of the adult comes from the cells of

the first branchial pouch, rather than from the hyomandibular (first

visceral) pouch, as a solid internal proliferation of cells from the upper

lining. At about the 12 mm. stage the sac-like outgrowths separate

THE FOREGUT 199

from the pouches, become invested with mesenchyme, and move to

the final position just posterior to the tympanic membrane and ven-

tral to the depressor mandibulae muscle. This gland is larger in the

younger frogs, attaining its maximum size in frogs of about 20 mm.

body length. It is a lymphoid gland, presumably a source of some

blood corpuscles, and apparently is essential to the early develop-

ment of the frog.

The Carotid Glands.

From the ventral ends of the first branchial (second visceral) pouch

there arise cell proliferations, at about the time the internal gills

appear (9 mm. stage) which develop into the carotid glands. These

glands develop and move to the junction of the internal and external

carotid arteries. Their function is to regulate the flow of blood, partic-

ularly that entering the internal carotid artery. This is accomplished

by means of its final spongy consistency. They may aid also in achiev-

ing adequate aeration of the blood.

The Parathyroid Glands (Pseudo-thyroid or "ventraler Krimenust").

From the ventral ends of the second and the third branchial (third

and fourth visceral) pouches are derived the parathyroid bodies, other-

wise known as the pseudo-thyroid bodies. In the adult these are

small, rounded, vascular glands which come to lie on either side of

the posterior portion of the hyoid cartilage.

The Ultimobranchial Bodies (Supra-pericardial or Post-branchial

Bodies).

The fourth pair of branchial (fifth visceral) pouches have no

glandular derivatives, but a pair of ultimobranchial (post-branchial)

bodies arise by cellular proliferations from the rudimentary fifth

branchial (sixth visceral) pouches. These organs arise as solid pro-

liferations of the pharyngeal wall and come to lie beneath the mucous

membrane of the pharynx lateral to the glottis. They shortly separate

from the pharynx and acquire cavities. The structure in the adult is

somewhat like that of the thyroid gland but the function has not

yet been determined.

The Thyroid Gland.

The thyroid is an endocrine gland which arises as a single median

thickening and evagination from the floor of the pharynx between the


base of the second pair of visceral arches, just before the time of

hatching and at about the 5 mm. stage. It later becomes a bi-lobed

and solid organ, far removed from its site of origin. Only its original

lining is endodermal, the bulk of the gland being mesodermal in

origin. At about the 10 mm. stage, however, it becomes separated

from the pharyngeal floor as a closed sac. This divides into two lobular

and vesicular masses, and moves to a position on either side of

the hyoid cartilage apparatus. The thyroid gland of the 15 mm.stage is divided but retains a connection at its anterior ends by a

short isthmus, so that it takes the shape of an inverted "Y," with a

short base. The wings move posteriorly, along the ventral face of

the hyoid cartilage, and these changes are correlated with gradual

changes in the hyoid apparatus. The glands enlarge, the surround-

ThyroidSucker Pharynx evagmation Hyoid arch Pharynx Thyroid

Thyroid

Hyoidcartilage

Notochord

Infundibulum

HypophysisHeart Hyoid Thyroid

cartilage glandTonguemuscles

Early development of the thyroid gland of the frog. (A,B) Separation of the

thyroid anlage from the pharynx. (C) Division of the thyroid anlage into lobes

at the 9 mm. stage. (D) Sagittal section of the 11 mm. stage showing the

position of the thyroid anlage in relation to the pharynx.

(Continued on facing page.)

THE FOREGUT 201

ing cartilaginous elements expand, the major blood vessels become

enmeshed in them, and eventually they come to lie close to the heart.

At the forelimb emergence stage of metamorphosis there is a marked

distension of the follicles, and by the tail resorption stage there is high

34^*'i*S?'*""

-vjl

Follicular

epithelium

Follicular

colloid

Partially vacuolated

colloid

Early development of the thyroid gland of the frog

—

{Continued) . (E) Earli-

est differentiation of the thyroid follicles. (F) Thyroid gland of the hindlimb

bud stage. (G) Same as "F" but entire gland at low power. (H) Thyroid gland

of the forelimb emergence stage. (I, J) Active follicular secretion of the thyroid

of the metamorphosing frog. (K) Thyroid gland of the frog immediately after

metamorphosis.


Thyroid gland at the time of metamorphosis. {Top, left) A portion of a

follicle at the tail resorption stage. Note the considerable number of para-

follicular cells. The cells adjoining the colloid show the highly elaborated Golgi

network. {Top, right) A follicle of the thyroid in a 3 mm. hindlimb length stage.

Note the homogeneous dense colloid and the heavy, compact Golgi material.

(Bottom) A large follicle of the thyroid of the forelimb emergence stage. Colloid

shows areas of various densities. Higher epithelium contains material showing

definite network and extensive ramification. (From S. A. D'Angelo & H. A.

Charipper, J. MorphoL, 64:355.)

THE FOREGUT 203

epithelium liquefaction while erosion of the colloid and follicular col-

lapse occur. At this time the genio-hyoid, hypoglossus, and sterno-

hyoid muscles are in close proximity to the thyroid, a situation which

does not persist to the adult. The gland shows hyperactivity during

metamorphosis with relative inactivity at the completion of the

metamorphic process. Its activity during these phases of development

is closely correlated with the development and function of the pitui-

tary gland, particularly of the basophilic cells in the pituitary gland.

In later stages it may be seen attached to the ventral aspect of the

hypoglossus muscle. The fully formed thyroid gland consists of sepa-

rate follicles, each made up of a single circular layer of cuboidal

(endodermal) epithelial cells, in the center of which is a lumen filled

with a colloidal mass.

The Tongue.

The tongue appears just before metamorphosis and is indicated as

an elevation in the anterior floor of the pharynx, just posterior to the

site of origin of the thyroid gland. Anterior to this the pharyngeal floor

is depressed and glandular, but during metamorphosis this glandular

area becomes the free anterior tip of the tongue.

The Lungs.

Shortly after the time of hatching, when the larva measures about

6 mm. in length, there appear bifurcating but solid cell proliferations

from the pharyngeal endoderm just behind the developing heart.

These soon develop into paired saccular evaginations, directed pos-

teriorly. These lung buds arise from the median ventral floor of the

pharynx at about the level of the rudimentary sixth visceral (fifth

branchial) pouch. The single short tubular connection of the lung buds

opens into the foregut through the glottis. The connecting column of

ceHs which has acquired a lumen, from which the lung buds arise,

will become the trachea. At the level of the glottis there is a short

transverse chamber known as the laryngeal chamber. The more pos-

terior bi-lobed mass eventually will open up as the primary bronchi

or lung buds. In the 1 1 mm. stage each lung bud will be surrounded by

peritoneal epithelium and will be invested with splanchnic mesen-

chyme. Later, each lung will become an ovoid, thin-walled, and

slightly alveolated sac, lined with endodermal squamous epithelium.

Outside of this epithelium, and constituting the substance of the lung,


are connective tissue, blood, and lymph vessels, all of mesodermal

origin.

The Liver.

The liver originates very early as a single median ventral endoder-

mal diverticulum which is directed posteriorly between the heart rudi-

ment and the yolk mass. The diverticulum enlarges slowly and its

anterior wall will thicken, become folded, and finally branched, to

form the liver proper. The ultimate lobes of the liver will retain their

tubular connection with the original diverticulum as the hepatic duct.

The original diverticulum will elongate as the bile duct leading to

a terminal vesicle, the gallbladder, which becomes very large in the

tadpole. All of these derivatives become invested with connective

tissue and blood from the splanchnic mesoderm, but some of the ad-

joining yolk cells become the true hepatic cells.

The Pancreas.

The pancreas arises as three rudiments at about the 9 mm. stage

in a manner somewhat similar to the liver. The first rudiment appears

as a single posteriorly directed ventral diverticulum of the bile duct

at its point of entrance into the foregut. This diverticulum soon

divides into two and the cellular elements grow around the bile duct

to fuse into a single mass of spongy tissue anterior to the bile duct.

Subsequently a third mass of similar tissue arises from the dorsal

wall of the gut, and attains junction with the original masses, all three

to form the much-lobulated pancreas. These three pancreatic rudi-

ments then use the single pancreatic duct which retains its original

connection with the gut, just posterior to the liver or hepatic duct.

It marks the boundary between the foregut and the midgut. The

cells of the islet of Langerhans arise early from endoderm.

The Oesophagus and Stomach.

After hatching, the undifferentiated portion of the foregut between

the glottis and the bile duct elongates to become the oesophagus. Dur-

ing early larval development this part of the gut becomes temporarily

occluded by an oesophageal plug of cells whose origin is unknown.

Its function during its brief appearance may be to help direct any

water and food from the mouth out over the gills. These early larvae

require no external source of food as they are supplied with abundant

THE MIDGUT 205

yolk. The oesophageal plug disappears by the 9 to 10 mm. stage of

development.

Beginning at about the 1 1 mm. stage, the oesophagus develops into

the very distensible organ of the adult while the stomach becomes

differentiated simply by a dilation of the next adjacent part of the

foregut. By the time of metamorphosis the stomach is somewhat more

distended than the oesophagus but it is otherwise indistinguishable

Rectum

Bladder

Cloaca

Anus

Digestive system and derivatives. (A) Before

metamorphosis. (B) After metamorphosis,

(Modified and redrawn from the Leuckart

wall chart.)

from it. In the newly metamorphosed frog, however, it assumes a

transverse position, shifting from the earlier longitudinal position,

and has all the characteristic layers of any vertebrate gut. This in-

cludes an inner and glandular layer of mucosa, intermediate layers of

connective and muscular tissue, and an outer covering of serosa.

The Midgut

The midgut is that portion of the original archenteron which is

found dorsal to the yolk mass as long as this mass exists, having a

roof and sides one cell in thickness. The floor is the thick yolk endo-

derm. After the time of hatching the yolk is rapidly absorbed. Be-

fore the time of metamorphosis the heretofore undifferentiated


tubular midgut becomes very much elongated into a doubly-coiled

tube which may be nine to ten times as long as the body of the tad-

pole. During metamorphosis, and while the tadpole changes from an

herbivorous (of the tadpole) to an omnivorous (of the frog) diet, this

midgut (potential small intestine) shortens to about three times the

length of the body, and its histology changes correspondingly. That

portion of the midgut directly posterior to the stomach becomes

the bent duodenum or duodenal loop. The small intestine, like the

stomach, is lined with a glandular mucosa and is supplied with both

circular and longitudinal (involuntary and smooth) muscles and an

outer serosa. It is suspended within the body cavity by a thin but

double layer of the peritoneal epithelium.

During the the earliest stages of development (2.5 mm. stage) there

may be seen a sub-notochordal or hypochordal rod of pigmented cells,

two or three cells in diameter, lying between the roof of the midgut

and the notochord. There is positional and structural evidence that

this column of cells is at some time associated with the roof of the

archenteron, from the level of the liver to the posteriorly placed

neurenteric canal. It becomes entirely free from gut and notochord by

the 4.5 mm. stage and disappears shortly after the time of hatching.

It has no known function. It may be of evolutionary and ontogenetic

significance only.

The Hindgut

This is the smallest portion of the original archenteron which lies

posterior to the yolk mass, between it and the posterior body wall.

The endoderm ventral to the closed blastopore evaginates to fuse

with the invaginating proctodeal ectoderm to form the anal plate.

This plate finally ruptures at about the 4 mm. stage to form the anus,

only the inner portion of which is lined with endoderm. The rectum

is the enlarged posterior end of the archenteron, and is therefore

endodermally lined. This does not develop until the time of metamor-

phosis.

Dorsal to the rectum are a temporary extension of the archenteron,

developed in consequence of the presence of the neurenteric canal,

and the posterior growth of the notochord. This is the post-anal gut.

At the 5 mm. stage only a remnant of the neurenteric canal can be

identified as parallel rows of pigmented cells extending ventro-poste-

riorly from the hindgut. This is the region of the disappearing post-

THE HINDGUT 207

anal gut. It represents the enteric portion of the temporary neurenteric

canal connecting the posterior ends of the neurocoel and archenteron.

It has no derivative in the adult, but has homologues in the develop-

ment of most of the vertebrates.

That portion of the hindgut between the rectum and the anus

forms an ectodermally lined chamber known as the cloaca. Into this

chamber empty the paired urogenital ducts, to be developed later.

Before metamorphosis there appears a ventral evagination of the

cloaca which gives rise to the bladder. This ultimately bi-lobed and

elastic organ has no direct connection with the excretory ducts as do

the bladders of higher forms. However, it is considered to be a reser-

voir, by way of the cloaca, for excretory fluids. In the closely related

toads, which live in hot sand, it also may be a reservoir for water

storage and an aid in respiration.



Endodermal Derivatives

The derivatives of the primary digestive tube, from anterior to posterior, are:

Mouth—with the exception of that part from oral aperture to point of the

internal nares (choanae)

.

Pharynx—dorso-ventrally flattened cavity into which mouth leads.

Visceral clefts—junction of endodermal lining of visceral pouch with ecto-

dermal invagination from side of head to form clefts associated with

visceral pouches II, III, IV, and V. Pouch VI never forms a cleft.

By the time the clefts are open they are covered by the operculum so

that they lead into the opercular chamber ventro-laterally. External

gills degenerating.

Thyroid gland—now separated from floor of pharynx, dividing posteriorly

into two lobes. Found just anterior to heart as paired pigmented

masses.

Thymus gland—derived from dorsal part of first and second visceral pouches

(first degenerating) and migrating posteriorly to position near the

ear.

Parathyroid glands (epithelioid bodies)—from ventral portions of third and

fourth visceral pouches. Difficult to locate.

Ultimobranchial bodies—small pigmented masses at level of trachea ventral

to sixth visceral pouches. Difficult to locate.

Trachea and lungs—laryngo-tracheal groove from median ventral floor of

pharynx near visceral clefts leads into tubular trachea which bifurcates

into two primary bronchi. These expand posteriorly into lungs.

Oesophagus—gut from laryngo-tracheal groove to stomach, bending behind

liver.

Stomach—identified by folded lining and thick muscular walls.

Duodenum—from pyloric end of stomach to coiled intestine.

Liver—highly vascularized and enlarged organ which incorporates vitelline

veins. Original liver diverticulum becomes gallbladder with commonbile duct (ductus choledochus) leading into dorsal wall of duodenum.

Pancreas—within curvature of stomach, duct joining common bile duct.

Intestine—two complete spiral coils posterior to duodenum. Cloaca as yet

unformed but pronephric ducts open into posterior end of hindgut.

CHAPTER THIRTEEN

Tlie Mesodermal Derivatives

The Epimere (Segmental or Vertebral The Reproductive System

Plate) The Gonoducts

The Somites The GonadsThe Vertebral Column The Hypomere (Lateral Plate Meso-The Skull (Neurocranium of the derm)

Adult) The Coelom (Splanchnocoel)

The Visceral Arches The Heart

The Appendicular Skeleton The Circulatory System

The Mesomere (Intermediate Cell The Arterial System

Mass) The Venous System

The Pronephros or Head Kidney The Lymphatic System

The Mesonephros or Wolffian Body The Spleen

The Adrenal Glands

The Epimere (Segmental or Vertebral Plate)

The Somites.

These dorsally located blocks of metamerically arranged meso-

derm are differentiated from the anterior to the posterior. The first

ones to be completed are located just posterior to the auditory cap-

sule. A total of 13 pairs is formed from the head to the base of the

tail. In the tail of the 6 mm. tadpole there may develop as many as

32 pairs of these transient somites, making a total of 45 pairs by the

6 mm. stage. The two most anterior or occipital pairs of somites

become indistinguishable and the larval tail somites are lost during

the process of metamorphosis. This leaves a total of 11 pairs of

somites from which develop most of the striated muscles and the

primary skeleton of the body. In the head and pharyngeal region the

mesoderm is in the form of loose mesenchyme.

Each somite develops in a characteristic manner, having an outer

thin layer of cells known as the dermatome or cutis plate and a mesial

mass of cells known as the myotome or muscle segment. Between

these is a cavity, the myocoel, which becomes elongated dorso-ven-

trally as the cutis plate extends to the body wall and appendages.

Originally this myocoel is continuous with the splanchnocoel or

body cavity. The myocoel eventually becomes obliterated. From the

209

210 THE MESODERMAL DERIVATIVES

Table of Somites, Vertebrae, and Related Nerves of the Tadpole(Elliott)

Cartilaginous

Elements in

Sclerotome

THE EPIMERE (SEGMENTAL OR VERTEBRAL PLATE) 211

jection. The sclerotome of the two potential posterior vertebrae fuses

to form the cartilage and then the bony urostyle which encloses the

posterior end of the notochord.

The dermatome or cutis plate gives rise to the dermal layer of the

dorsal and lateral skin, to connective tissue between the myotomes,

and to the musculature of the limbs. The dermal layer of the ventral

body wall arises from the somatopleure, so that while the dermis be-

comes continuous it originates from two sources.

The myotomes or muscle segments of the early larva arise as rather

solid aggregations of cells, concentrated at the level of the spinal

cord and notochord. They enlarge at the expense of the contained

myocoels. Before the time of hatching, these myotomal cells become

elongated and their muscle fibrillae orient their axes in the longitudi-

nal direction of the embryo. This indicates a shift in the axes from the

original direction in the early myotomes. The myotomes become

separated from each other by septa or myocommata (connective

tissue sheets). These are derived from the cutis plate, and assume a

"V" shape with the apex of the "V" pointing posteriorly. These myo-

tomes become the layer of striated or voluntary (voluntarily con-

trolled) muscles of the back, limbs, and dorsal body wall. The fibers

of many of these muscles are arranged ultimately in a variety of direc-

tions to provide for complete body control.

The muscles of the heart, blood vessels, and viscera arise from the

hypomeric splanchnopleure and are known as smooth or involuntary

muscles.

Limb buds develop from the accumulation of loose mesenchyme of

adjacent somites surrounded by ectoderm. Within this blastema of

somatic mesoderm there arise muscle and bone, developing from the

body outwardly. The forelimb muscles develop before those of the

Relation of the Anterior Somites to Nerves

Head Somite


hindlimbs, although the hindlimbs are the first to emerge during the

process of metamorphosis.

The sclerotome arises at loose cells which are proliferated off from

the ventro-medial portion of the myotome of the somite. They will

give rise to the entire axial skeleton, as described below. By this time

(5 mm. stage) the somites are entirely separate from the lateral plate

mesoderm.

The Vertebral Column.

The development of the notochord has been described already. Its

origin cannot be attributed exclusively to any of the three germ layers.

Its cells remain few in number, become flattened antero-posteriorly,

and are highly vacuolated. The notochord then becomes surrounded

by (1) the primary or elastic sheath derived from the notochordal cells,

(2) the secondary or fibrous sheath, and (3) eventually an outer skele-

togenous sheath which is of mesodermal origin. This latter connective

tissue sheath encapsulates the nerve cord and also extends laterally

between the myotomes by the 15 mm. stage. The notochord finally

disappears in the frog. It is partially replaced by and partially con-

verted into material of the centrum of each vertebra. Neural arches

and transverse processes develop outwardly from the cartilaginized

skeletogenous sheath.

The vertebral column of the frog consists of ten vertebrae of which

the last is modified into an elongated rod-like bone known as the uro-

style, mentioned above. The first vertebra is the cervical atlas, fol-

lowed by seven abdominal vertebrae and finally the ninth or sacral

vertebrae. The urostyle takes the place of the caudal vertebra. The

skeletogenous sheath from the sclerotome encloses the spinal cord

and notochord by the time of hatching, and by the 15 mm. stage

some of this sheath has given rise to cartilage. It is within this cartilage

that the vertebrae develop. The formation of this cartilage, due to

the accumulation of the sclerotomal cells between the myotomes, is

segmented in a manner which alternates with the developing muscle

segments. However, at all levels the spinal cord and notochord will

be enclosed in this double ring of connective tissue which is being

transformed progressively into cartilage and finally into bone. Around

the notochord it is known as the perichondrium and around the

spinal cord as the vertebral cartilaginous arch.

Ossification begins, and the cartilage gradually is displaced by an


inward growth of true bony cells known as osteoblasts. Eventually the

notochord itself becomes invaded by these bone-forming cells and is

transformed into the bony centrum of the vertebra. Each centrum

becomes concave anteriorly, for the reception of the convex projec-

tion of the more anterior centrum. At the level of the lateral myo-

tomal mass, connective tissue invades the notochord to form the inter-

vertebral discs or ligaments of hyaline cartilage. Each disc splits into

an anterior and posterior part, and each becomes ossified and later

fuses with the corresponding part of the adjacent centrum. There are

connecting ligaments which arise from the original sclerotome along

both the dorsal and the ventral faces of the centra. They alternate in

position with the myotomal segments.

The ossified cartilage which surrounds the spinal cord is known as

the neural arch. Both the anterior and the posterior margins of each

neural arch bear a pair of short zygapophyses which are processes that

articulate and tend to join successive vertebrae. Spinal nerves emerge

from the spinal cord through intervertebral foramina, between the

sides of the succeeding neural arches.

Dorsal to each potential vertebra surrounding the spinal cord is a

single median cone of ossification which becomes the spinous or

neural process of the vertebra. The successive neural spines are con-

nected by ligaments. The paired transverse and bony processes arise

laterally, at right angles, from the cartilage of the centrum and be-

come continuous with the minute bony ribs. No transverse processes

or anterior zygapophyses are developed on the most anterior vertebra,

the atlas. This vertebra is modified to articulate with the occipital

condyles of the skull. The transverse processes of the ninth vertebra

are elongated and are directed obliquely posterior to provide attach-

ment for the ilium of the pelvic girdle. The tenth vertebra is greatly

modified into the single tubular urostyle and is derived from the

sclerotome (skeletogenous material) of the last two somites of the

body.

The Skull (Neurocranium of the Adult).

The skull of the adult frog, defined as that part of the skeleton

which surrounds and supports the brain and special sense organs,

is composed of relatively more cartilage than is the skull of most higher

vertebrates. The cranial cartilages are an exception to the general

rule as to origin, and are derived from ectodermal neural crests.


The frog skull represents a type intermediate between the skulls

of lower fishes and higher reptiles. It is made up of the cranium,

which arises from the cartilage and is therefore known as the chondro-

cranium; the sense capsules; a portion of the notochord; and a visceral

skeleton, which includes the jaws and the hyoid apparatus and mem-brane and dermal bones. There are cartilage (endochondral) bones

Mental

Meckelian

Basibranchial

Ceratohyal

Ceratobrani h

Hypobronchia

Lower jaw, Hyoid

end Branchials

Notochord

Trabeculae cornu

Ethmoid plate

luscular

process

-Anterior

process

Trabecula

Paloto-

~quadrate

Posterior

process

.4/ Basilar plate

Parachordals

Skull and

upper jaw

Rana pipiens embryonic chondrocranium. Dor-

sal view of the 11 mm. stage. Neurocranium is

solid, labeled to the right. Splanchnocranium is in

outline, labeled to the left. (After Gaup, from

Ziegler.)

which arise by the ossification of cartilage and there are membrane(dermal) bones which develop from (dermis) superficial membranes

without the intervention of a cartilage phase. These latter bones are

sheet-like and can be stripped from the cartilage of the true cranium.

The bony floor of the cranium arises largely from the notochord

in conjunction with a pair of cartilaginous (sclerotomal) rods knownas the parachordals at about the 7 mm. stage. These are arranged

longitudinally on either side of the notochord with which they becomefused to form the parachordal plate or floor of the cranium. Thebasilar plate is immediately articulated with the notochord and is

THE EPIMERE (SEGMENTAL OR VERTEBRAL PLATE)

Orbito-nosal foramen

Auditory

capsule-

215

cartilage

Paloto-quadrate

cartilage Posterior maxillary

process

Polato-quadrate

(anterior ascend-

ing process)

Anterior maxillary

process

Palato-quadrate

(pterygoid process)

Olfactory cartiloge

Anterior

maxillary process L'l

Pre-maxillary bone

Nasal bone

Maxillary bone

Pterygoid bone

Palato-quadrate

(ascending pro

cess)

Palato-quodrqte,

{pterygoid~'

'

process)

Proof ic

foramen

Annulus .-|^

tympanicus

Palato-quadrate

cartilage (orticu

process)

Auditory capsule

Skull of Rana during metamorphosis. (A) Chondrocranium of R. fusca

during metamorphosis, lateral view. (B) Skull of R. fusca after metamorphosis,

dorsal view. The membrane bones are shown removed from the left side. (After

Gaup, from Ziegler.)

Plectrunn

Quadrato-

jugular

bone

united with the parachordals on either side. The cartilaginous ele-

ments of the skull appear by the time of hatching and form what is

known as the chondrocranium. These elements are not displaced by

bone until about the 30 mm. body length stage.

Anterior to the parachordal plate, and continuous with it, is an-


Other pair of rods of cartilage and then bones. These are the trabeculae

(or trabecular cartilages) which join mesially to form the ethmoid or

intranasal plate. The anterior space between the trabeculae is the

basicranial fontanelle within whose cavity the infundibulum tempo-

rarily lodges.

The sense capsules are simultaneously formed. Each auditory cap-

sule forms from mesenchyme which surrounds the developing inner

ear. It consists of mesotic and occipital cartilages. The occipital carti-

lages form the side and roof of the auditory capsule and skull and the

mesotic forms the floor. Between the occipital cartilages of the two

sides is the large posterior opening for the spinal cord to the brain,

known as the foramen magnum. Anteriorly the trabeculae form the

orbital bones around the eyes. The anterior extremity of these carti-

lages are the trabecular cornu which form the bony olfactory capsules.

Anteriorly they fuse with the labial or suprarostral cartilages.

Some of the parts of the cranium are covered by thin plates which

originate in the dermis and not from the sclerotome and are there-

fore called dermal bones. Many of these cover open spaces of the

chondrocranium and line the mouth. They appear very early and

include the fronto-parietals, nasals, premaxillae, maxillae, quadrato-

jugals, squamosals, parasphenoids, vomers, dentaries, and palatines.

The main cartilage bones are the exoccipitals, prootics, stapes, eth-

moids, parts of the pterygoquadrates, articulars, hyoids, branchials,

and mento-Meckelians.

The Visceral Arches.

These arches give rise to the visceral skeleton (splanchnocranium)

but arise as six pairs of vertical condensations of mesenchyme lateral

to the embryonic pharynx. After the mouth opens each of these arches

develops cartilage. The most anterior pair, the mandibular arches,

give rise to the dorsal palatoquadrate bone and to the ventral Meckel's

cartilage. The palatoquadrate (pterygoquadrate) cartilage arises from

the maxillary (dorsal) portion of this first visceral arch and joins the

posterior end of the trabeculae. The palato portion gives rise to the

bulk of the upper jaw and the quadrate portion to the annulus tym-

panicus of the middle ear. The Meckel's cartilage gives rise to the core

of the lower jaw or the dentary bone. The large ceratohyal cartilages

appear posterior to the Meckelian cartilage in the hyomandibular

arch. The basicranial (copula) is an unpaired cartilage found between


I

Development of the thyroid gland and related hyoid cartilages. ( 1 ) Diagram-

matic representation of the developing thyroid of the frog in relation to the

hyoid cartilages in early metamorphosis; (HCP) hyoid cartilage proper,

(BH) basihyoid cartilage, (BB) basibranchial cartilage, (HB) hyobranchial

cartilage, (1) approximate initial site of the thyroid anlage, (2) approximate

position of the thyroid at the hindlimb bud stage, (3) approximate position of

the thyroid at the fully differentiated hindlimb bud stage.

(2) Same as "1" except at later stages of metamorphosis; (4) Approximate

position of the thyroid at forelimb emergence, (5) approximate position of the

thyroid at the tail resorption stage. (D'Angelo and Charipper: 1939.)

the two large ceratohyals. The more anterior basihyal is slow to de-

velop cartilage but can be identified by the U-shaped thyroid gland

which appears directly ventral to it. The jaws are suspended to the

skull by a cartilaginous bone which develops from membranes.

Portions of the second visceral (hyoid) arch give rise to the hyoid

cornu. Parts of the third and fourth visceral arches (first and second

branchials) give rise (at the 9 mm. stage) to the plate-like hypo-

branchial cartilage apparatus of the adult. The four pairs of slender

ceratobranchials extend posteriorly from the hypobranchials. None

of the arches remain as such by the time of metamorphosis, the only

remnants being the derivatives just listed. The frog tadpole does not

possess true teeth, but does have ectodermally covered horny "jaws"

and so-called teeth. Oral papillae, appearing as teeth, constantly are

wearing away and bear no relation to the bones with which the

more functional teeth of the post-metamorphic frog will be associated.

At metamorphosis dermal papillae appear on the upper jaw, associ-

ated with the vomers, and are known as tooth germs. The overlying

epidermis is then involved in transforming the papillae into pseudo-

teeth, with a minute amount of dentin and enamel. The teeth are for

holding rather than for macerating living food, and never are de-

veloped as highly as in reptiles or mammals.


The Appendicular Skeleton.

The pectoral girdle consists of the scapuia, coracoid, and precora-

coid, the last two to be replaced by the clavicle. Except for the clavicle,

which is dermal, this girdle arises from ossified cartilage that is calci-

fied only at the base, and articulates with the elongated scapula. The

epicoracoid (or precoracoid) and the xiphisternum remain as cartilage

while the single sternum, paired coracoids, and single episternuni be-

come ossified. The sternum arises from the fusion of a longitudinal

pair of cartilages which never join the ribs but remain both anterior

and posterior to the pectoral girdle. The humerus, or upper portion of

the forelimb, articulates with the glenoid cavity of the pectoral

girdle.

The pelvic girdle is a V-shaped fused mass of bone which supports

the hindlimbs and is associated with the urostyle. The paired anterior

ends of the pelvic girdle (ilium) are united with the enlarged trans-

verse process of the ninth or sacral vertebra. Other bones of this girdle,

arising from cartilage, are the ischium and pubis. The pubis alone

does not ossify. The ilium forms a cup, the acetabulum, which re-

ceives the head of the femur of the hindlimb. The bones of both

anterior and posterior girdles and limbs arise by the ossification of

preformed cartilage (i.e., endochondral in origin). There is evidence

that appendicular muscles of some amphibia may develop in situ

from somatic mesoderm.

The Mesomere (Intermediate Cell Mass)

Early in embryonic development the upper level of the lateral

plate mesoderm becomes separated off from the ventral sheets of

mesoderm, to give rise to the excretory and parts of the reproductive

systems. This is known as the intermediate cell mass or mesomere

because of its position relative to the other masses of mesoderm. It

is also known as the nephrotome because it gives rise to excretory

units, both the larval pronephros and the functional mesonephros of

the frog. The derivatives of this mesoderm will now be described;

the description of the development of the hypomere and its deriva-

tives, the coelom and circulatory system, will be deferred until later

in the book.

THE MESOMERE (INTERMEDIATE CELL MASS) 219

The Pronephros or Head Kidney.

The somatic wall of the nephrotomal region at the level of the

second, third, and fourth somites thickens and projects laterally be-

tween the hypomere and the body ectoderm. This longitudinal bulge

is known as the pronephric shelf, because it is due to the development

of the pronephros or head kidney and can be seen readily from the

exterior. This is noticed first from the external view at the tail bud

stage (2.5 mm. body length) as the pronephric ridge or bulge just

posterior to the gill plate. Within the lateral extensions of these

nephrotomal masses there appear cavities, the nephrocoels, which

run together longitudinally to form a common tube. This tube and

its lumen grow posteriorly along the lateral border of the nephro-

tomes, to join the cloaca and be known as the paired pronephric or

segmental ducts. Actually the duct is not in any way segmented, but

it will receive segmental kidney tubules. Posterior to the fourth somite

this duct develops independently of the nephrotomal tissue and be-

comes joined to the cloaca at the 4.5 mm. stage.

Three pairs of pronephric tubules, one at the level of each of the

somites II, III, IV, appear between the segmental duct and the

coelom. The original connection of the nephrotomes with the somites

is lost in favor of a nephrogenic cord. Finally each tubule acquires an

opening, the nephrostonie, into the adjacent coelom. This opening

shortly acquires cilia and becomes funnel-shaped, by the time it

reaches the 5 mm. body length stage. The pronephric tubules lengthen

and become coiled (convoluted) and at the 6 mm. stage are embedded

in the mesenchyme and sinuses of the developing posterior cardinal

Internal glomerufus

Pnmory kidney tubule

Pronephrrc duCt

Ciliated

nephrostome

Externalglomerulus

Development of the pronephric tubule. (Schematized after Felix.)


vein. The entire pronephric mass is surrounded by the connective

tissue of the pronephric capsule, derived from the adjacent somatic

mesoderm and myotome.

Between each nephrotome and the dorsal aorta, suspended into the

body cavity and surrounded by splanchnic mesoderm, a very rudi-

mentary capillary network known as the glomus arises from the

dorsal aorta. This appears to be an abortive attempt to develop a

glomerulus characteristic of the adult mesonephric kidney, and occurs

in response to inductive influences from the non-functioning proneph-

ric tubules.

The pronephros is structurally much like the functional kidney of

the frog, except that it is much less complex. It acquires a rich vas-

cular supply, both arterial and venous, but has no outlet. It is doubt-

Peritoneol funnels(persistent nephrostomes)

Persistent nephrostomes oi the frog. (A) Ventral face of the frog kidney

showing the persistent peritoneal (nephrostomal) funnels which drain the

body cavity fluids directly into the venous sinuses. Note the nearby adrenal tissue.

(B) High power surface view of the kidney. (C) Section through a peritoneal

(nephrostomal) funnel.


ful that it ever functions as an excretory organ, but it attains its

maximum development at about the 12 mm. stage. Along with the

glomus the elongated and coiled tubes of the pronephros constitute

a formidable mass of tissue which projects into the dorsal body cavity

and is surrounded largely by peritoneal epithelium. As the pronephric

mass expands, it fills the posterior cardinal sinus and is bathed in

venous blood. As the lungs enlarge and grow against the pronephros,

the splanchnic mesoderm covering the lungs and the pronephros is

brought together to form a trap or pocket which is merely a reduced

coelomic chamber into which the temporary nephrocoels open. This

is the pronephric chamber. It remains open both anteriorly and

posteriorly into the lung chamber. A pronephric capsule is formed by

an overgrowth of myotomic mesenchyme and an upfolding of somatic

mesoderm from the lateral plate. Thus a connective tissue sheath is

formed around the embryonic head kidney.

By the 1 1 mm. body length stage the pronephroi are large and

conspicuous bodies with blind tubular outgrowths; but by the 20 mm.stage they have begun to degenerate and neither the tubules at this

level nor the related glomi remain. The mesonephric kidney develops

rapidly and takes over the increasingly important excretory functions.

The Mesonephros or Wolffian Body.

The nephrotomal mass posterior to the pronephros and mesial to

the segmental duct gives rise to the mesonephros or Wolffian body

which is the functional kidney of the adult frog. It extends from the

level of somites VII through XII and begins to develop at the 8 to

10 mm. stage.

In this region, as in the more anterior regions, there are segmental

nephrotomes which merge to form a pair of continuous nephrotomal

masses from the seventh to the twelfth somites. The mesonephros is

therefore of both somatic and splanchnic mesodermal origin. At the

level of each of these somites there develop several nephrotomes, each

with a separate nephrocoel. The nephrocoels at this level are knownas the mesonephric vesicles which become convoluted and constricted

so as to give rise to primary, secondary, and tertiary units, in that

consecutive order. It is obvious, therefore, that the kidney units of

the mesonephros are more complicated than those of the pronephros,

from the very beginning of their formation. They are not actually

metameric.


Wolffianesonephric)

duct

•enal gland

Urogenital system of the male frog during amplexus, showing vasa efferentia

white with spermatozoa. Note the mesorchium and huge posterior vena cava.

The primary mesonephric vesicle gives rise to a dorsal evagination

which becomes the secondary unit of the mesonephric vesicle. Fol-

lowing this there is another evagination from the ventro-lateral side

of the primary vesicle, toward the segmental duct with which it be-

comes continuous. This is known as the inner tubule of the tertiary

mesonephric vesicle. It becomes greatly coiled and encroaches upon

the developing and adjacent cardinal vein. The inner tubules become

the tubular portion of the adult kidney. The segmental duct from this

level posteriorly is known as the Wolffian or mesonephric duct, and

this will remain as the excretory duct or ureter.

A third evagination, the outer tubule, develops ventrally from

the mesonephric unit. The cavity of this tubule joins the coelom by

way of the nephrostome. However, by the 20 mm. stage this unit has

broken away from the rest of the mesonephric unit and has changed

its connection from the mesonephros to the lateral division of the

cardinal vein. This vessel becomes the renal portal vein. This con-

nection can be demonstrated very easily in the recently metamor-

phosed adult frog where persistent ciliated nephrostomes can be seen

on the ventral face of the kidneys and which can be shown to main-


tain direct connections with venous capillaries of the kidneys. The

outer tubule aids in forming Bowman's capsule around each glomeru-

lus. The ciliated nephrostoine then arises along with the mesonephric

unit, develops a normal coclomic connection, and then shifts its re-

lationship to become associated with blood sinuses of the posterior

cardinal vein. This occurs within the kidney, at about the 15 mm.stage. The original nephrostomes remain in the adult frog on the

ventral face of the kidney as ciliated peritoneal funnels which are

readily seen. These convey coelomic fluids directly into the blood

sinuses of the kidney. They might still be regarded as accessory excre-

tory structures. More nephrostomes appear than can be accounted for

in the above manner and it is not known whether the extra ones arise

by splitting of the original ones or by additional peritoneal invagina-

tions to the mesonephric tubules.

The mesonephric tubules which remain to give rise to the urinifer-

ous tubules of the functional and adult mesonephric kidney are elon-

gated considerably. They form spherical masses around developing

blood capillaries emanating from both the renal artery (from the

dorsal aorta) and from the renal portal vein. These form capillary

networks which are both arterial and venous and constitute true

glomeruli, having no connection with the coelom as do the glomi

of the pronephric level. Each glomerulus is surrounded by the thin-

walled Bowman's capsule of the fully formed Malpighian body (renal

corpuscle) within the mesonephric kidney. By the time of metamor-

phosis this mesonephric kidney is fully formed and functional, ready

to assume the increased excretory load of a terrestrial organism.

The Adrenal Glands.

Most of the endocrine glands are derived from the pharyngeal

endoderm or brain ectoderm, but the adrenals are derived from both

ectoderm and mesoderm. They are a complex of two endocrine glands

and are not related functionally to the excretory system but can be

described conveniently at this time because of their proximity of

origin and their final position in the adult. In the adult frog this com-

posite gland appears as a thin layer of yellowish brown granular

material on the ventral face and closely adherent to each kidney.

It is composed of both a medullary (suprarenal) substance and a

cortical (interrenal) substance, so called because of their positional

relationship in the higher forms rather than in the frog.


The Adrenal Cortex (Interrenal Gland). The cortical cells

of the adrenal gland may be seen as early as the 10 mm. body length

stage, and probably arise from the coelomic mesothelium. They are

aggregated into round or elliptical bodies lying on the dorsal surface

of the coelomic cavity, just ventral to the dorsal aorta. The original

mass measures about 0.8 mm. in length. It contains a number of

nuclei embedded in a syncytial mass of acidophilic tissue. The con-

tained granules may be pink, yellow, or black. By the 12 mm. stage

Cortical cells

Adrenal gland of the recently metamorphosed frog. Camera

lucida drawing illustrating the detailed nature of an islet from

the interrenal region. (From Stenger and Charipper: 1946.)

paired gonad primordia may be seen, also in the dorsal coelomic

epithelium but in a more lateral position. By the 13 mm. stage the

cortical masses have doubled. They form a cap around the base of

the upper four-fifths on either side of the dorsal aorta, just posterior

to the site of formation of a single aorta. At this stage the cortical ma-

terial may be seen intermingled with, or in close proximity to, the

gonad anlage.

By the 37 mm. body length stage the adrenal cortex consists of

lobed masses of cells surrounded by the notochord, the body wall,

and the Wolffian duct below. Groups of cortical cells are, by this

time, encapsulated and come into intimate association with the mes-


onephros. By metamorphosis these groups of cortical cells are inter-

spersed with strands of medullary cords. The groups migrate from

the dorso-mesial side of the mesonephros to its ventral surface where

they are found in the adult, extending almost the full length of the

kidney. It now consists of clear-cut areas of medullary tissue associ-

ated with circumscribed lobes of cortical tissue. This cortical tissue

characteristically contains distinct Stilling cells, never to be found in

the medullary tissue. We must conclude, therefore, that since the

Adrenal cortical anlage at the 10 mm. stage of the frog

tadpole, showing the syncytial nature of the mass and the pres-

ence of pigment granules, (From Stenger and Charipper: 1946.)

frog adrenal contains both cortical and medullary material its origin

is from both mesoderm and ectoderm.

The Reproductive System.

The Gonoducts: The Male. In the male the mesonephric or

Wolffian duct acts as both an excretory organ (i.e., ureter) and a re-

productive duct (i.e., vas deferens) since all spermatozoa pass from the

testis into the uriniferous tubule of the mesonephric kidney by way of

the vasa efferentia. These tubules are modified anterior mesonephric

tubules which convey spermatozoa into the Malpighian corpuscle by

way of a permanent opening into the Bowman's capsule. The sperm

then continue to the cloaca by way of the uriniferous tubule and the


mesonephric duct. There is therefore no separate vas deferens, this

mesonephric or Wolffian duct acting in the normal capacity of a vas

deferens. In the male frog, then, it is a true urogenital duct from the

beginning.

As the mesonephric (Wolffian duct or vas deferens) duct reaches

the cloaca it enlarges and becomes somewhat glandular, and is knownas the seminal vesicle. It is within this vesicle that the sperm masses

are retained during amplexus and from which they are ejected into

the water during oviposition.

The Miillerian duct, which is the male homologue of the oviduct,

develops late in embryogenesis from a longitudinal ridge of cells

which project into the coelomic cavity just ventral and lateral to the

pronephric region and the Wolffian duct. This ridge sometimes de-

velops a tube and extends from the cloaca to a point near the junc-

tion of the lungs, anterior liver lobes, and heart. It is covered with

and suspended by a thin layer of peritoneal epithelium. Originally

this Miillerian duct was described as originating from a longitudinal

splitting of the segmental duct, but this has been doubted recently.

The duct often degenerates in the male adult but there may be vestiges

in higher vertebrates such as the appendix, testis, and prostatic utricle.

Its homology to the female oviduct can be demonstrated by treating

the male with estrogens which cause it to hypertrophy, and to have

the appearance of an oviduct.

The Female. The mesonephric or Wolffian duct of the female func-

tions exclusively as a ureter. The oviducts arise in a manner similar

to the Miillerian ducts of the male but they acquire a lumen which is

surrounded by ciliated and glandular epithelium and muscular walls.

It is suspended to the dorsal body wall by a double fold of peritoneum.

The anterior end of each oviduct consists of a persistent group of

nephrostomes which become the fimbriated and highly elastic infundib-

ulum or ostium tuba. The oviduct shows slight differential gland

development and convolutions from the anterior to the posterior,

where it becomes the very distensible uterus. The two uteri open sepa-

rately into the cloaca.

The body cavity of the female develops an abundant supply of

cilia, in response to the appearance of an ovarian hormone. These

cilia may therefore be regarded as secondary sexual characters of

the female, appearing shortly after the time of metamorphosis.

The Gonads. The paired gonads arise as a single median sex cell


ridge dorsal to the midgut at about the time of hatching (6 mm.stage). These cells presumably arise from the yolk sac splanchno-

pleure, or from the gut endoderm. Some evidence in favor of this

theory is the fact that the primordial germ cells are heavily laden with

yolk. As the two lateral mesodermal plates converge above the gut,

this group of cells migrates to a position directly dorsal to the dorsal

mesentery, between the posterior cardinal veins at the 8 to 9 mm.stage. Since this ridge of primitive gonadal material is rather long,

it is now called the sex cell cord. This shortly divides longitudinally

into two, and each part moves ventro-laterally to a position between

the developing Wolffian body and the dorsal mesentery projecting

into the body cavity as the genital ridges. Each mass will enlarge and

project ventrally into the coelomic cavity, carrying with it a covering

of peritoneal epithelium. This membrane becomes double and sus-

pends the gonad to the body wall as the gonad grows into the coelom.

NEUROCOEL

NOTOCHORD

MESONEPHRO: WOLFFIAN DUCT

GONAD

MESONEPHROS

COELOM

STOMACH

HEPATIC PORTAL VEIN

EXTERNALGILLS

PERITONEUM

LIVER

INTESTif

Schematized diagram through the level of the gonad primordium of the 1 1 mm.frog tadpole.


Posterior veno cava

3lood corpuscles

Mesonephric

(Wolffian)

duct

Germinal epithelium

Primordial germ cell

Coelom

Gonad onlage

Dorsal mesentery (splonchnopleure)

— Presumed path of primordial germcells to gonad anioge

Gonad primordia of the 1 1 mm. frog tadpole.

This double membrane is known as the mesorchium in the male and

the mesovarium in the female.

These coelomic projections of pre-gonad material are known as

genital ridges. Sex is, of course, not distinguishable as the gonad

primordium shows no particular cellular differentiation at this time.

The general organization of both presumptive ovary and testis is one

of an enlarged sac with germ cell primordia around the periphery

and the primary genital cavity in the center.

The rete cords now appear as strands of cells which arise from

the developing Malpighian bodies of the mesonephros and grow into

the primary genital cavity of the undifferentiated gonad. These

strands unfortunately are referred to in some textbooks as sexual

cords or sex cords, but these terms are confused too easily with sex

cell ridge or sex cell cords, terms just used. They properly are called

rete cords.

Sex differentiation occurs before the time of metamorphosis, at

about the 30 mm. body length stage. The gonads show macroscopic

differences at this time. In the case of the potential ovary the cells

around the periphery of the gonad primordium multiply and the rete

cord strands, which have invaded the genital cavity, begin to develop

their own cavities, which are known as the secondary genital cavities.

These become confluent and form from seven to twelve large ovarian

sacs, each connected with the others. The cellular material of the

rete cords then gives rise to clusters of cells, one of which begins

to grow toward the primary oocyte stage. The others (primordial

germ cells) become the surrounding nurse or follicle cells. All these


A.

Primordial germ cells (g.c.) in the tadpole of the common frog {Rana

temporaria). (A) In the mesentery (m.). (B) In the genital ridge; (g.ep.)

germinal epithelium; (/.c.) follicle-cell. (Courtesy, Jenkinson: "Vertebrate

Embryology," Oxford, The Clarendon Press.)

cells probably are derived from the outer germinal epithelium. They

are found at all times as clusters of potential ovarian follicles, both

nurse cells and ova. Many primordial germ cells are expelled from

the forming follicles and disintegrate within the peritoneal cavity.

In the testes the rete cord material remains condensed to give

rise to germ cells which migrate from the periphery to the gonad

primordium, to form cysts. These cysts give rise to the seminiferous

tubules of the newly metamorphosed male frog in which all stages

of spermatogenesis will develop. Each seminiferous tubule is con-

nected with a collecting tubule and vasa efferentia, all within the rete

cord material. Some of the rete cord cells give rise to the interstitial

(endocrine) and connective (stroma) tissue of the testis.

Fat bodies develop from one-third to one-half of the anterior ends

of either of the genital ridges. These are storage masses built up dur-

ing the summer feeding periods in anticipation of hibernation and the

subsequent active spring breeding period. In closely related toads of

certain species the anterior end of the testis often may develop a

rudimentary ovary, with oocytes, which will respond to hormonal

treatment much in the manner of a normal ovary. This is known as

Bidder's organ. Isolated oocytes have been found in the seminiferous


Sections through the gonads of Anurans before and after differentiation.

{Top left) Section through the gonad of a 30 mm. tadpole. Note the migration

of sex cord elements into the gonad, and the appearance of the primary genital


THE HYPOMERE (LATERAL PLATE MESODERM) 231

tubules of the otherwise normal male frog, and hermaphrodites have

been described. These facts simply emphasize the fundamental similar-

ity of the frog ovary and testis, both in origin and in early develop-

ment.

The Hypomere (Lateral! Plate Mesoderm)

The Coeloni (Splanchnocoel).

This splanchnocoelic cavity arises as irregular spaces in the lateral

plate mesoderm, ventral to the mesomere. These spaces become con-

fluent to form a continuous split, the coelom. The cavity therefore

develops between an outer somatic and an inner splanchnic layer of

mesoderm. The earliest appearance of any coelomic space is in the

tail bud stage, and in the anterior heart mesenchymal area. There

shortly develop three continuous divisions of the coelom, the muscle

coelom (myocoel of the somite), kidney coelom (nephrocoel of the

intermediate cell mass), and gut coelom (splanchnocoel of the lateral

plate). In addition, there is the heart coelom or pericardial cavity.

By the time of hatching, the coelomic space (splanchnocoel) on


cavity, pc, filled with embryonic connective tissue. {Top center) The gonadof a 40 mm. larvae, showing the maximum development of the morphologically

undifferentiated germ gland. The solid sex cords completely fill the gonad.

( Top right) Section through a young ovary. The secondary genital cavity, sgc,

is small. The germ cell nests, gen, are disappearing. The germ cells are in

early stages of pseudoreduction. (Center left) The ovary of a 70 mm. larvae,

showing the obliteration of the egg nests and secondary genital cavity. A fewresidual oogonia persist at the periphery of the gland, ro. (Center right) Thegerm cells have passed from the peritoneum into the sex cords in this gonadand are surrounded by a follicular covering of peritoneal and sex cord elements.

(Bottom left) Section through a gonad, showing the first signs of differentiating

into a testis. Large irregular primary genital spaces filled with embryonic con-

nective tissue are present. At ct are cords of cells passing out to germ cells in

the epithelium. (Bottom center) Young testis, showing the secondary genital

cavity at sgc and two germ cells enclosed within a cross tubule from the sex

cords at ct. Note the formation of other cross cords growing out to the germinal

epithelium. (Bottom right) Testis showing the nests of germ cells—anlagen of

testis ampullae. Note that the sex cord cells have become organized into vasaefferentia at the hilus and short tubules pass out to enter the ampullae. (From'The Germ Cells of Anurans," by W. W. Swingle. Reprinted from /. MorphoLPhysiol, 41, No. 2, March 1926.)


cardial

one side merges ventrally with that on the other side (except at the

extremities of the embryo) to form a horseshoe-shaped cavity sur-

rounding the yolk and enteron. The only region where the coelom

does not become continuous is the area dorsal to the enteron (gut)

or ventral to the notochord. At this region the splanchnic mesoderm

comes together to form the dorsal mesentery. This double sheet of

splanchnic mesoderm then acts as a suspensory mesentery for the

gut, and it continues around to enclose the enteron and the yolk. This

lining of the visceral cavity and covering of the gut will become the

peritoneal epithelium.

Just posterior to the pharynx the coelomic space extends into

the heart mesenchyme as the pericardial cavity, to be described below.

During the tail bud stage, for in-

stance, one could inject a colored

dye into the coelom at the yolk level

and this would be carried anteriorly

to fill the forming pericardial cavity.

The frog develops lung respiration

by the time of metamorphosis but it

does not develop a diaphragm to

separate the coelom further into a

pleural and peritoneal cavity as in

the case of higher vertebrates. These

cavities remain continuous, in the

frog, as the pleuro-peritoneal cavity,

but the pericardial cavity becomescut off entirely from the embryonic coelom. This is accomplished by

the aid of the developing ductus Cuvieri which helps to form the

septum transversum at about the time of metamorphosis.

The Heart.

The heart rudiment develops early from splanchnic mesenchymal

cells which grow ventrally toward the mid-line below the pharynx.

There they aggregate to form paired spaces (anterior coelomic spaces)

at about the tail bud stage. This seems to occur under the inductive

influence of the archenteric floor. In the meantime there appear

scattered and possibly endodermal cells dorsal to the point of junction

of the converging mesodermal masses and ventral to the pharynx.

These are known as endothelial cells which become endocardial cells.

The coelom and its derivatives in

the frog.


It will be remembered that the mesoderm of the ventral lip of the

blastopore is at first continuous with the floor endoderm of the gut

by way of the yolk. It may be suggested that the separation of the

heart endothelial cells may be a later (hence more anterior) phase

of the same separation of endoderm and mesoderm. It is purely an

academic consideration whether the endocardium is endodermal or

mesodermal in origin, for it arises from original endoderm in a manner

similar to most mesoderm. The converging sheets of mesoderm then

fuse above and below, enclosing these endothelial cells. The wall of

the inner tube thus formed becomes greatly thickened during this

convergence and is known as the myocardium. It will begin to give

rise to the striated, syncytial, and involuntary muscles of the heart

by the 5 mm. body length stage of development. The enclosed endo-

thelial cells will form the lining of the heart known as the endocar-

dium. The cavity surrounding the myocardium is the pericardial

cavity which itself is enclosed by an outer (splanchnic) pericardium.

The pericardial portion of the embryonic coelom is separated from

the peritoneal cavity by growth of the common cardinal veins, the

liver, and peritoneal folds, all of which comprise the septum trans-

versum. This process is completed during metamorphosis.

As in the gut region there will develop a double-layered suspensory

membrane of the heart known as the dorsal mesocardium. This re-

mains attached to the heart at the anterior and posterior extremities

of that organ, but ruptures at the level of the elongating and expand-

ing heart itself. There is also a temporary ventral mesocardium,

which arises before the dorsal mesocardium. It soon ruptures through

to provide a single continuous enclosing pericardial cavity completely

surrounding the heart except for the previously described dorsal

mesentery.

The mesocardium then disappears both dorsally and ventrally at

the level of the heart, and this is accomplished in part by the rapid

growth and expansion of the heart itself. The heart arises by the

bilateral downgrowth of mesenchyme which becomes a single short

tube. This tube finally lengthens and becomes divided to form the

coiled, folded, and three-chambered heart of the adult.

The early differentiated heart consists of a simple tube which be-

comes divided into three chambers. The fused pair of vitelline veins

becomes the meatus venosus which empties into the first division of

the heart, the sinus venosus. Then follows (more anteriorly) the thin-


walled atrium, the thick-walled ventricle, and finally the bulbus

arteriosus (bulbus aortae) which leads to the ventral aorta or truncus

arteriosus.

The development of the heart is accomplished within the limited

space provided by the surrounding body tissues and organs, so that

this organ becomes a reversed "S" coiled tube. With the embryo fac-

ing to the right, and looking at it from the right side, the shape of

the heart would be something like this:

Vitelline vein

0. Dorsol view

-Vitelline vein

B Dorsol view.

Vitelline vein

D. Ventrol view

Development of the amphibian heart.

The posterior part of the original tube becomes folded dorsally

and anteriorly to the more anterior part of the original tube. Con-

versely, the anterior part (which is destined to become the muscular

ventricle) is folded ventrally and posteriorly to the other parts of the


heart. There is thus a reversal in the original anterior-posterior re-

lations of the heart in respect to the embryo. The extremities of the

heart, those parts which are held in place by the dorsal mesocardium,

are the posterior sinus venosus and the anterior truncus or bulbus

arteriosus. Anteriorly the bulbus arteriosus gives rise to three pairs

of arteries (aortic arches) to each of the three pairs of external gills.

Later a fourth pair is added. In the frog there are no aortic (arterial)

arches in either the mandibular or hyoid (first or second visceral)

arches.

Before the development of the heart has progressed this far (i.e.,

the 4 mm. stage) there appears a pair of blood vessels in the ventro-

lateral splanchnic mesoderm known as the vitelline veins. These are

associated closely with the yolk mass, hence the name "vitelline" from

the vitellus (yolk). They probably are derived, as is the heart, by

a separation of yolk endoderm and ventral mesoderm. They grow

anteriorly by the merging of blood islands or lacunae until the veins

merge, anterior to the yolk and liver mass, and become the fused

vitelline veins. This point of fusion of the vitelline veins is called the

sinus venosus from the very beginning. It represents the most poste-

rior limit of the heart. The part of the heart into which the sinus

venosus empties is the atrium or the undivided embryonic auricles.

The atrium and subsequent auricles remain thin-walled and become

very elastic. Shortly (i.e., at the 7 mm. body length stage) there grows

down from the roof of the atrial chamber a longitudinal sheet of endo-

cardium which is known as the interatrial septum. This divides the

atrium into a right and left auricle, and the sinus venosus now leads

into the right auricle. Eventually the left auricle will receive blood

from the pulmonary veins.

The ventricle or anterior portion of the original heart tube becomes

very thick-walled and muscular and is never divided longitudinally

into two as it is among mammals. There is a transverse division

groove between the atrium and the ventricle at the folds in the in-

verted "S" coil just described. The ventricle is therefore folded cau-

dally and ventrally to the atrium and leads into the truncus arteriosus

which is suspended to the dorsal body wall by the permanent dorsal

mesocardium at this level. The truncus arteriosus becomes partially

subdivided by a vertical and longitudinal septum (the spiral valve)

and gives rise to the paired ventral aortae which are connected with

the various aortic arches.


The Circulatory System.

The blood vessels arise from the fusion or confluence of blood

islands which develop first in the splanchnic mesoderm and later in

scattered mesenchyme. These are isolated groups of yolk-laden cells

which form endothelial pockets containing blood-forming cells. These

lacunae ("litde lakes") merge to form the blood vessels, and eventu-

ally the embryo acquires a complete and closed blood vascular sys-

tem, continuous with the heart. This occurs even before there is any

rhythmic pulsation of the blood or the heart. The blood islands give

rise to the early embryonic blood, but this blood is later derived from

the spleen and bone marrow and possibly the liver.

The Arterial System. The dorsal aorta arises in splanchnic

mesoderm dorsal to the enteron. Above the pharynx it is double and

the vessels are known as the supra-branchial aortae or dorsal aortae.

Aortic arches develop within the mesenchyme of the visceral arches

(III to VI). They arise first as blood islands which merge to form

blood vessels. The afferent branchial portion of each arch joins the

ventral aortae, while the efferent branchial portion joins the dorsal

aortae. This junction does not occur in either the mandibular or the

hyoid arches, but it does occur in all the others. A closed circuit is

established, therefore, between the ventral heart and the dorsal aortae

by way of the viscerally located aortic arches.

The third, fourth, and fifth visceral arches (branchial arches I, 11,

and III) now give rise to secondary blood vessels or capillary loops.

These connect dorsally (efferent branchial artery) and ventrally

(afferent branchial artery) with the corresponding aortic arch. The

intermediate section of each new vessel then grows out into the devel-

oping external gill. That portion arising from the ventral part of the

aortic arch, and therefore nearest the heart, is then known as the

afferent branchial loop. The returning portion from the gill which is

connected with the dorsal part of the aortic arch is the efferent bran-

chial loop. These loops coil through the filaments of the developing

external gills, but the original portion of the aortic arch in each of the

visceral arches remains for a brief time. The blood may therefore

flow through either of two courses (i.e., the arch or the branchial

loop), during the early functioning of the external gills. Usually no

external gill grows outward from the sixth visceral arch and as a

result an external branchial vessel never develops. The aortic arches

THE HYPOMERE (LATERAL PLATE MESODERM)

. Rhombocoel

Notochord

,

Otic vesicle.

Dorsal aorta

Dorsal part

aortic arch

237

Endocardium

Myocardium

Pericardial cavity

- Pericardium—

Ventral part of

aortic arch

Larval respiration in the frog. {Top) Development of the external gills.

{Bottom) Changes from internal to external gill circulation.

tend to disappear or become relatively non-functional, as the external

gills handle the major volume of the blood flow in this region.

As the external gills degenerate there develops a short-circuited

connection between the afferent (ventral) and the efferent (also ven-

tral) ends of the branchial vessels. This probably is aided by the

remnant of the original aortic arches. This short circuit supplies the

newly forming internal gills. All of the blood from the ventral aorta


Dorsal oorto

Anterior cardinal vein (jugular)

I-

Internal carotid ortery

Left vitelline

vein


Sinus venosus

External carotid artery

Ventral aorto

Bulbus arteriosus

Ventricle

Dorsal aorta

Pulmonary vein

Pulmo-cutanfous orr


Ductus cuvieri (jugular)

Cutaneousartery /

Pulmonary artery

Posterior vena cava

Hepatic veiri

External carotid

Ventral aorta

Bulbus arteriosus

Ventricle

Sinus venosusRight ouricle

Blood vascular system of the developing frog embryo. (Top) Early embryo,

from the right side. {Bottom) Late frog embryo, from the right side.

then must pass through the filaments of the internal gills until the

lung circulation develops. These gills degenerate at the time of meta-

morphosis.

The paired dorsal aortae extend into the head and the aortic arches

of the first pair of branchial arches (visceral arch III), after the loss

of branchial respiration, remain to carry blood into the head from

the heart. These become the large anterior or internal carotids, orig-


inally the anterior extensions of the dorsal aortae. They proceed to

the base of the skull and give rise to the palatine (to the roof of the

mouth), the cerebral (to the brain), and the ophthalmic (to the eyes)

arteries. The original paired ventral aortae proceed into the head as

the smaller external carotids and join the blood islands and sinuses

of the floor of the mouth and the tongue as the lingual arteries. The

carotid glands, derived from the third visceral arch and having the

Never develop

Internal carotid

External carotid

Verteoral artery

I-/— Dorsal aorta

Fate of the aortic arches of the frog embryo.

function of filtering carotid blood, are located at the bifurcation of

the internal and the external carotids. This is at the original ventral

origin of the first branchial arch.

The aortic arches which pass through the second branchial (fourth

visceral) arch become greatly enlarged and retain their connection

with the dorsal aortae on both sides, to become the main systemic

trunk. That portion of the dorsal aorta anterior to the second bran-

chial arch then degenerates, so that all of the arterial blood of the

first branchial arch must pass anteriorly and all of the blood of the

second branchial arch must pass posteriorly. The systemic trunk on


each side gives rise to the laryngeal artery ( to the larynx and muscles

of the hyoid apparatus) and the oesophageal arteries (to the shoulder

and brachial arteries of the limb). The paired systemic arteries join

within the body to form the single dorsal aorta, sometimes called the

descending aorta, which in turn gives rise to many branches. These

include the renals to the glomi of the pronephros and glomeruli of

the mesonephros; the large coeliac artery to the stomach, liver, pan-

creas, and intestine; the gonadals; the lumbars; and finally the iliacs

to the hind legs.

The aortic arch which develops into the third branchial arch (fifth

visceral) disappears. Those in the fourth branchial arches, however

(sixth visceral), develop connections with blood islands in the skin

and the lungs to become the pulmo-cutaneous arteries. In the frog,

the skin represents about 60 per cent of the necessary respiratory sur-

face. Some time after metamorphosis the connection of this fourth

pair of branchial aortic arches severs connections with the dorsal

aorta (now known as the systemic trunk), leaving only a vestigial

strand of tissue known as the ductus Botalli or ductus arteriosus. In

this manner the systemic arteries to the viscera and hmbs are sepa-

rated from the respiratory arteries to the skin and lungs.

The arterial circulation of the frog is not entirely efficient for the

simple reason that each truncus arteriosus is divided only partially

by two septa into three channels, and some of the recently aerated

blood from the lungs will be sent again to the lungs. One of the chan-

nels leads to the carotid arches; a second joins the systemic arches;

a third leads from the right side of the heart, carrying venous blood to

the pulmo-cutaneous arches. The largest exit is to the systemic trunk,

and, until the time of metamorphosis, the two main anterior arteries

must pass through the gill circulation where the blood can adequately

exchange its carbon dioxide for oxygen. The systemic trunks give rise

to the pharyngeal arteries, which supply the mandibular arches, and

to the subclavians, which supply the forelimbs, and then they join

to form the single dorsal aorta. This large vessel then gives off several

thoracic and lumbar arteries to the back, a large coeliac artery and

smaller mesenteric arteries to the viscera, iliac arteries to the limbs,

and a caudal artery to the tail.

The Venous System. The vitelline veins are the first blood vessels

to develop and they appear as blood islands ventro-lateral to the yolk

mass. The fused anterior vitelline veins comprise the meatus venosus,


later to become the hepatic vein emptying into the sinus venosus. The

right posterior vitelline vein shortly disappears and the left one re-

mains as the hepatic portal vein, bringing blood from the viscera to

the liver. Shortly after hatching, a hepatic vein arises from the liver

substance to pass directly to the sinus venosus via the path of the

original anterior vitelline vein. After the development of the posterior

vena cava the two main liver lobes direct their separate hepatic veins

into the sinus venosus.

The paired common cardinals grow obliquely in a dorso-lateral

direction from the sinus venosus toward the body wall and then divide

to send branches anteriorly and posteriorly. The paired outgrowths

of the sinus venosus are known as the ducts of Cuvieri or Cuvierian

sinuses. The anteriorly growing extension of the common cardinal

vein is the anterior cardinal vein. This receives blood from the tongue,

hyoid, thyroid, parathyroid, and the floor of the mouth by a branch

known as the external (inferior) jugular vein. It also receives blood

from the brain, shoulder, and forelimb by way of another branch, the

internal (superior) jugular vein. A third contributing vessel is the

subclavian vein which brings blood from the brachial vessel of the

forelimb and the cutaneous vein of the skin. All of these vessels con-

tribute to the original anterior cardinal vein and at their point of

junction are known as the anterior or superior vena cava. Since the

posterior cardinal veins fuse mesially and become separated from the

common cardinal, the anterior vena cava becomes the only remnant

of the embryonic ductus Cuvieri.

The posterior cardinal veins are directed posteriorly from the

paired common cardinals (ductus Cuvieri). Each of these forms a

sinus around the temporary pronephric tubules and then grows pos-

teriorly as the single median cardinal vein between the mesonephroi

and into the tail. During its course it receives vessels from the dorsal

body wall.

The posterior cardinals go through considerable change during

the pre-metamorphic development of the frog. There is degeneration

of the entire left cardinal and most of the right cardinal veins. Thesections posterior to the pronephric sinuses tend to fuse mesially to

form the single posterior vena cava (post-caval vein or inferior vena

cava) which empties directly into the sinus venosus, having movedposteriorly from the ductus Cuvieri. The paired hepatic veins, from

the liver lobes, are now incorporated into the enlarged posterior vena


cava as it enters the sinus venosus. These changes leave the ductus

Cuvieri connected with only the anterior or superior vena cava, which

may now be called the pre-caval vein.

Posteriorly the vessel produced by fusion of the posterior cardinal

veins, now considered as the median cardinal vein, is enlarged as

COMMON CARDINAL V.

PRE CAVAL V.

SINUS VENOSUS

RIGHT AURICLE

INFERIOR VENA CAVA^(POST. VENA CAVA)

ANTERIOR PART RIGHTPOST. CARD. DEGENERATES

INTERNAL JUGULAR V.

EXTERNAL (SUPERIOR)JUGULAR V (ANT CARO.V.)

INTERAURICULAR SEPTUM

LEFT AURICLE

ATRIO VENTRICULAR VALVE

VENTRICLE

'^PULMONARY V.

I

DEGENERATING ENTIRELEFT POST CARD. V

'KIDNEY

SUBCARDINAL(RENAL PORTAL) V,

VITELLINE {HEPATIC PORTAL) V.

Blood vascular system of the frog tadpole. Venous system, ventral view.

( Continued on f(icini> pa(ê.)


sinuses which shortly become organized into a vessel. This vessel con-

nects with the hepatic vein through a new junction anterior to the

liver. The median vein remains as the posterior vena cava to receive

blood from the tail, which degenerates at metamorphosis, and from

the kidneys (several short renal veins). After metamorphosis there is

no remnant of this vessel posterior to the kidneys, since the tail dis-

appears. Thus the post-caval vein is composed of several elements: a

hepatic vein derived from the original left vitelline vein; a short por-

tion of the right posterior cardinal vein; the median channel derived

from the fused right and left posterior cardinal veins; and a short

new section just posterior to the hepatic section, which grows pos-

teriorly.

The lateral pair of vessels on the outer margin of the mesonephros

brings blood from the hindlimbs (sciatic and femoral) by way of the

common iliac, and from the dorsal body wall (lumbar). These are

known as the renal portal veins and terminate in the mesonephroi.

The pulmonary veins arise at about the 6 mm. body length stage as

posterior dorsal outgrowths of the sinus venosus to the lung buds.

ANTERIOR CEREBRAUINTERNAL CAROTIO

EFFERENTBRANCHIAL ART

eULBUS ARTERIOSUS

LINGUAL'VISCERAL \

VISCERAL III: BRANCHIAL I

VISCERAL IV: BRANCHIALII

VISCERAL V> BRANCHIAL III

PUL. CUT. ARTERX^

CUTANEOUS ARTERY /«j

i gLOMuS

PULMONARY ARTERY

VISCERAL Vl -BRANCHIAL IV

SYSTEMIC TRUNK'DESCENDING AORTA

Blood vascular system of the frog tadpole

—

(Continued). Arterial system,

ventral view.


Olfactory pit

Lateral append

Epiphysis Anterior choroid plexus

outhCornified embryonic

teeth

Diocoel Diencephalon

^ qr u':d coot

ic'ino

-Lens

-Cornea

-Pharyr-

Mandibulor muscles Embryonic teeth

4

Hyoid cartilage

Dperculor chamber

nfundibulum

Pigmented loyer

, Optic cup

Retina

Optic nerve

Trabecular cartilageRhombencepholon

""^\ Piluitory

Hypophysis 1

Cranial nerve V

Cranial nervesVII and VIII

Opercular Sucker

chamber

Hypobronchiol plate

Hyoid cartiloge ^1^^,^,^ gi^^d

Anterior tip of pericordiol cavity

Serial transverse sections of the 11 mm. frog larva.

These veins eventually fuse and open directly into the left auricle,

bringing blood from the lungs after metamorphosis.

The single abdominal vein arises as a pair of veins growing poste-

riorly from the sinus venosus along the ventral body wall to the level

of the cloaca, making junction with the femoral veins of the hind

legs. These paired vessels then fuse, anterior to the cloaca, into a

single median ventral abdominal vein. Tt then loses its connection


Cranial nerve VII

Posterior choroid plexusEndolymphatic duot

Honzontdl ^,^^.Anterior semi-circular senni-circular >*'^^si,-Rhombocoel

canal canolAnterior semi-

Gill rakers

Aortic arch-'

Bulbus arteriosus

Dorsal aorta -,^

Aortic orch

NIolochord

Anterior \

lymph space \

Opercular chambei

Pericardial cavity

- '' ,' ijrW.'^'" y-y-^ Ventricle

"-•'Ôpercular channber


Auditory capsule

Internal jugular ^' ji:-^'

ternol carotid artery

Auricle

Pericardial cavity

Ventricle

lymph spoce-tt4- ('i.

Gill filaments

Internal gills _ .

Gill chamberPericardial cavity

/ Ventricle

Ventral mesocardium

Posterior cordinol vein

Lung bud-*

Neurocoel Spinal cord

IMotochordWolffian duct

^5t,,j^Lung bud

Spirocle'

Posterior coelom

14 15

Serial transverse sections of the 11 mm. frog larva.


with the sinus venosus and establishes a new connection with the

hepatic portal vein, originally the right vitelline vein, which enters the

liver lobes.

The Lymphatic System. The lymphatic system of the frog is

made up of very large lymph spaces in various parts of the body, with-

out well-defined connecting lymph vessels such as occur in mammals.

However, all of the four lymph hearts are protected by valves so that

the lymph always passes from a lymph heart to a lymph or blood ves-

sel, and never in the reverse direction. There are no muscular coats to

the lymph spaces, but they are lined with flat endothelial cells and

connective tissue. The connective tissue forms septa which divide the

lymph spaces and attach them to the underlying muscle.

Between the third and fourth somites between the peritoneum and

the integument, at the time of hatching (6 mm. body length), there

are developed crude anterior lymph hearts, surrounded by muscle

fibers. These are connected by an ill-defined pair of vessels just be-

neath the skin, the subcutaneous lymph spaces, one of which extends

anteriorly and the other posteriorly, each to merge with the venous

plexuses. The anterior paired lymph vessels send extensions ventrally

to the branchial region considerably after the time of hatching. Paired

thoracic ducts develop from the anterior lymph hearts (at the 26 mm.stage) just between the posterior cardinal vein and the dorsal aorta.

The posterior vessels give rise to both dorsal and ventral vessels in the

larval tail. After metamorphosis posterior lymph hearts appear in

association with the intersegmental veins near the hindlimbs. As pre-

viously described, there is flow of lymph from the peritoneal cavity

into the venous sinuses of the kidney by way of the ciliated peritoneal

funnels of that organ.

The Spleen.

This hematopoietic organ arises in the vicinity of the stomach at

about the 10 mm. body length stage as a cluster of cells within the

dorsal mesentery. By the 15 mm. stage the spleen is seen as a definite

projection from the mesentery, covered by coelomic epithelium. Ap-

parently some wandering cells from the intestinal epithelium invade

the spleen at this time. By the 30 mm. stage these cells become reticu-

lar and are vascularized and can be recognized as splenic cells, and

the organ as the ovoid spleen of the adult.



Mesodermal Derivatives

Mesenchyme—loose, primitive mesoderm found scattered throughout tadpole.

There are condensations in head region where cartilage-forming cen-

ters are developing, later to give rise to neurocranium except dura and

pia mater, which come from neural crest. Connective and blood vas-

cular tissues are also to be derived from mesenchyme. Around meso-

dermal notochord are sclerotomal (mesenchyme) cells which will

form axial skeleton.

Epimere—most dorsal mesodermal masses appearing as metameric somites,

the most anterior of which are being transformed. About 12 somites in

trunk and 32 in tail remain.

Dermatome—distinct band of mesenchymatous cells lying dorso-laterally

just beneath ectoderm, to form dermis (cutis).

Myotome—central portion of somite which is being organized into muscle

bundles, divided in tail region into dorsal and ventral bundles.

Sclerotome—thin layer of mesenchyme cells surrounding nerve cord and

notochord from which will develop axial skeleton.

Mesomere—intermediate mesodermal mass from which urogenital system is

derived.

Pronephros—primary, embryonic kidney made up of a few coiled tubules

and ciliated nephrostomes.

Pronephric ducts—lateral to dorsal aorta and dorsal to posterior cardinals,

these ducts lead from pronephros posteriorly to join each other just

before they fuse with cloaca.

Mesonephros—large mass of nephrogenic tubules, without nephrostomes,

forming permanent frog kidney.

Gonads—cell masses hanging into coelom from dorsal peritoneum between

mesonephros and gut, in vicinity of lung buds.

Hypomere—lateral plate mesoderm, ventral to mesomere, and consisting of

somatic and splanchnic layers with intermediate coelomic cavity.

Pericardial cavity—surrounds heart, not yet separated from true peritoneal

cavity.

Peritoneal cavity—body cavity, surrounding viscera.

Mesenteries—double-layered dorsal mesentery supports viscera. A remnant

of ventral mesentery (gastrohepatic omentum) is found between stom-

ach and liver.

Spleen—accumulation of cells lateral to mesenteric artery in dorsal mesentery.

Circulatory System—differentiated as heart, arteries, veins, lymphatics, and

contained corpuscles.

Heart—already a three-chambered structure as in the adult.

Sinus venosus—junction of post-caval and common cardinals, leading to

right atrium.

Atria—thin-walled, paired heart chambers, at least partially separated from


each other by interatrial septum. Right atrium receives systemic veins

while left atrium receives pulmonary vein from lung buds.

Ventricle—thick-walled, muscular, and generally ventral to atria. Leads

into truncus and bulbus arteriosus.

Bulbus arteriosus—tubular extension of early embryonic heart which leads

directly into aortic arches.

Truncus arteriosus—short, ventrally directed vessel leading into bulbus

arteriosus. (Syn., ventral aorta.)

Arteries—major arteries only.

Afferent branchial arteries.

Lingual (external carotid)—extensions of ventral aorta into lower jaw.

First branchial within third visceral arch.

Second branchial—forked portion of first branchial found within fourth

visceral arch.

Third branchial—temporarily found within fifth visceral arch; degen-

erates.

Fourth branchial—within sixth visceral arch.

Efferent branchial arteries.

Anterior cerebral (internal carotid)—gives rise to basilar artery and com-

missural artery passing beneath infundibulum to other side of head (see

diagram). Extensions of first branchial artery from within first visceral

arch.

Second branchial—within fourth visceral arch, enlarges and grows pos-

teriorly as descending aorta to join corresponding vessel of other side

to form systemic trunk. Junction about level of liver.

Third branchial—within fifth visceral arch; degenerates.

Fourth branchial—within sixth visceral arch, loses its connection (ductus

arteriosus) with descending aorta and grows posteriorly to form pul-

monary and cutaneous arteries (see diagram).

Dorsal aorta—single large artery formed by junction of two descending

aortae (radices aortae) and extending posteriorly into tail, giving off

various smaller arteries en route.

Intersegmental arteries—paired and metameric arteries which grow from

dorsal aorta dorso-laterally between myotomes.

Glomi—short branches from radices aortae which grow toward pronephric

chambers. Really undeveloped glomeruli.

Mesenteric artery—single large vessel growing ventrad from dorsal aorta

to supply viscera. Later becomes coeliac artery.

Caudal artery—extension of dorsal aorta into tail.

Veins—major veins only. Generally walls are thinner than arterial walls.

Cardinal system.

Anterior cardinals (superior jugulars)—irregular in cross section, found

in head, closely associated with internal carotid arteries.

Inferior jugulars—bring blood from lower jaw, accompanying external

carotid arteries, and join common cardinals.


Posterior cardinals—ventro-lateral to dorsal aorta, dorsal to nephro-

genic tissue. Carry blood to common cardinals.

Subcardinals—smaller veins, ventral to mesonephric tissue, and bring-

ing blood from caudal vein. Become renal portals.

Common cardinals—ducts of Cuvier which receive blood from anterior

and posterior cardinal vessels, and pronephric sinus, and convey it to

sinus venosus at its postero-lateral margin.

Post-caval vein—posterior vena cava or inferior vena cava. This vessel

has grown posteriorly from hepatic vein and incorporates mesoneph-

ric level of right posterior cardinal vein. It therefore passes through

liver into sinus venosus ventral to entrance of right common cardinal

vein (see diagram).

Hepatic system—derivatives of original vitelline veins.

Hepatic vein—fused vitelline veins carrying blood from liver sinuses to

the sinus venosus. Short and difficult to distinguish from post-caval

vein to which it gives rise.

Portal vein—common vessel receiving blood from pancreas, stomach,

intestine, etc., representing original vitelline veins.

Lymphatic system—not well developed at this stage.

Chondrocranium—cartilage centers may be found, within which bone will de-

velop in formation of the embryonic skull.

Neurocranluni—skeleton around brain and primary sense organs.

Anterior to posterior.

*Labial (supra-ostral) cartilages—paired, short, within upper lip and

lateral to mouth.

*Cornua trabecularum (trabecular horns)—paired cartilage bars which

accompany olfactory tubes.

Ethmoid (internasal) plate—fused trabeculae within median line, anterior

to ascending process of pterygoquadrate.

*Pterygoquadrate (palatoquadrate)—derived from maxillary portion of

first visceral arch and joins posterior end of trabecula by its posterior

ascending process. At its anterior end joins anterior ascending process.

Basicranial fontanelle—skeletal cavity for pituitary gland.

Trabeculae—paired cartilage bars on either side of basicranial fonta-

nelle, converging slightly anteriorly.

Basilar plate—single, fused cartilage mass immediately anterior to noto-

chord. Joins parachordals and trabeculae.

Parachordals—lateral to notochord, at its anterior end.

Splanchnocranium—derived from the visceral arches. Anterior to posterior.

Mental (infra-rostral) cartilages—paired, but meet in mid-line of lower lip.

*Meckelian cartilages—laterally projecting flanges to which muscles will be

attached joining pterygoquadrate.

*Ceratohyals—large cartilages arising within hyomandibular arches, posterior

to Meckelian cartilages.

*May be ectodermal.


=^Basibranchial (copula)—single cartilage between ceratohyals.

*Hypobranchials—paired, posterior to ceratohyals, and derived from third

visceral arches. Articulates also with basibranchial.

*Ceratobranchials—three or possibly four slender processes within third to

sixth visceral arches, extending from hypobranchial on each side.

*May be ectodermal.

APPENDIX

Ckronoloîcal Summary of Orân Anlaên

Appearance or Rana pipiens

Stage 17: 3 mm. total body length.

Blastopore closes, neurenteric canal transient, neural tube closed

but no differentiation of central nervous system, body elongated with

back curve changing from convex to concave, myotomes evident but

movement by surface ciliation. Internally most of major organ sys-

tems begin to appear including heart mesenchyme, visceral arches,

and lateral plate mesoderm, pronephros, hypochordal rod, sense and

gill plates, optic vesicles, and both auditory and olfactory placodes.


First evidence of muscular movement, primary brain differentia-

tions, infundibulum and hypophysis still separate, lateral line nerve

develops from vagus placode, auditory placode becomes separated

from head ectoderm, lens placode formed, ventral roots develop from

spinal cord, hypochordal rod separated from gut, notochord becomes

vacuolated and acquires an elastic and a fibrous coating from sclero-

tome, oesophageal plug formed, dorsal aorta paired anteriorly and

single posteriorly, vitelline veins prominent and functional.


Epiphysis well developed as evagination of prosencephalon, in-

fundibulum, and hypophysis are approximated in position, thyroid

evagination develops, trigeminal nerve from first cranial crest and

placode, facial and auditory nerves from second cranial crest and

placode, lens vesicle separated from head ectoderm, 45 pairs of

somites developed (by 5.5 mm. stage), 32 of which are in tail, muscles

with fibrillae, sclerotome abundant, heart differentiated and beating.

251

252 CHRONOLOGICAL SUMMARY OF ORGAN ANLAGEN


Rhombocoel

Auditory capsule (cartilage)

Cranial nerve VIII

Semicircular canals (inner ear)

Stapedial plate

Columella

Annulus tympanicus

Tympanum

Middle ear vestibule

Perilymph space

Eustachian tube(hyomandibular pouch'

Head mesenchyme

Cranial cartilage

Heart mesoderm

Parts of the middle and inner ear of the frog, schematized drawing.


Hatches, oral suckers at maximum development, blood system well

developed, including endocardium, myocardium, and pericardium

of the heart, pulmonary veins develop but do not function, gill circu-

lation abundant, blood islands numerous, lymph hearts first appear

at level of somites III and IV, pronephros with glomi and segmental

duct become partially embedded in posterior cardinal veins, a solid

rod of ectoderm cells extend from olfactory pit to pharynx, four cra-

nial ganglionic masses appear, optic nerve develops from neuroblasts

of retina, and sympathetic nervous system arises from neural crests.


Transition from larva to tadpole, mouth open, cornea transparent,

cerebral hemispheres and vesicles differentiated, endolymphatic duct

open to surface, heart folding upon itself, paired and metameric

dorsal root ganglia, interatrial septum separates auricles, sex cell

(genital) ridge appears.

CHRONOLOGICAL SUMMARY OF ORGAN ANLAGEN 253


Heart partitions completed and heart differentiated, circulation

reaches tail fin, lung buds develop, sub-notochordal rod disappears

and mesonephros begins to form.

< .iC^- -- --^

Olfactory organs of Rana pipiens tadpoles at 20 mm. total body length.

(af) Anterior choanal fold,

(cc) Choanal canal,

(ct) Cornu trabeculae.

(cv) Christa ventralis.

(ec) Entrance canal,

(ig) Inferior canal.

(ir) Inferior recess.

(b) Lateral appendix.

(Ig) Lateral groove.

(pf) Posterior pharyngeal fold.

(pr) Posterior recess.

(sc) Superior cavity.

(Courtesy, Ruth Cooper, /. Exper. ZooL, 93:415.)

to IS aI II III IV V

Stages in the metamorphosis of Rana pipiens. (Stage I) The oral sucker elevations

have completely disappeared. Four rows of labial teeth are present, one pre-oral andthree post-oral. Chromatophores have appeared and become numerous on the dorsal

and lateral surfaces, and extend progressively ventrad. The limb bud is visible as a

faintly circumscribed elevation in the groove between the base of the tail and the belly

wall. The height of the elevation is less than half the diameter of the disk. {Stage H)The height of the limb bud elevation (i.e., the length of the bud) is equal to half ofits diameter. The first row of post-oral labial teeth is usually at the middle to form a

pair of crescents. On the dorsal surface of the head the lateral line system is becomingconspicuous as pigment-free lines, especially in darkly pigmented individuals. Themelanophore patches covering the gill region on either side usually meet in a narrowband ventral to the heart. (Stage 111) The length of the limb bud is equal to its

diameter, and it continues to grow both in length and diameter at an approximatelyequal rate. This stage is relatively long in duration. (Stage IV) The length of the limbbud is equal to one and a half times its diameter. (Stage V) The length of the limbbud is twice its diameter. The distal half of the bud is bent ventrad. There is no flatten-

ing of the tip. (Continued on facing page.)

ISA-

256

13 w ^ cj ? &< <U OA

257

3 *-

g OH

"^ xi "S 5c -^ u

<u .S >- X)> -^ ^ ^

« •£ J= <"

M O^£ e

a <U X!

••*. C/3 ^ _

|> CTJ (L) V5 C

"53 73 r-- M-. g]

.5 s ^ I ;=

258

S =^^

C <U ri"" o y

<u - rt ^^

o c^

5 >-ra n> i^

- 1>1)^ -

.—

>

C3 cj^^ «"*" tA D.

>< u «

oi; rt a.^ y o

y & •

3 C C

X) —£ '3

S b

00 «

5 't-o

E -5 e

lU - 3— Oo

s s--Ô (U <L)

H 3

c «U >,^ 3 <«

CHRONOLOGICAL SUMMARY OF ORGAN ANLAGEN 259


Mouth developing with embryonic teeth, opercular fold begins

backward growth, tongue anlage appears, carotid glands develop

from ventral wall of first branchial pouch, epithelioid bodies from

wall of second and third branchial pouches, oesophageal plug disap-

pears, pancreas anlage is formed, pharyngeal arteries as well as sub-

clavians grow from dorsal aorta, mesenteric and lumbar arteries ap-

pear, parachordal plate develops in floor of cranium, brain develops

anterior choroid plexus, optic lobes, and cerebellum, cranial nerves

(except abducens) well developed including all parts of trigeminal.


Atrophy of mucous glands, mouth with horny jaws and cornified

lips takes in vegetable diet, intestine long and looped, operculum

closed on right side, thyroid bifurcates and separates from pharynx,

spleen anlage appears, two or three branchial clefts break through,

mesonephros developing, utricle separated from saccule by oblique

partition, all semi-circular canals indicated, optic lobes formed, me-

dian cardinal vein appears, and posterior vena cava begins to develop.


Cilia lost except on tail, operculum completed, cornification of

horny rasping papillae of jaws and lips, oesophagus differentiated

from stomach, lung buds invested with mesenchyme, pronephros

large and conspicuous, retina develops rods and cones, olfactory ap-

paratus develops lateral appendix, internal choanae open, olfactory

nerve is derived from olfactory lobe neuroblasts, and abducens is

formed from neuroblasts of medulla.

12 mm. total body length stage:

Olfactory tube develops complete lumen from external nares to

internal choanae, thymus separates from dorsal part of first branchial

pouch and comes to lie posterior to tympanic membrane near mandib-

ular muscles, maximum development of the pronephros which de-

generates by 20 mm. stage, cortical (interrenal) adrenal arises from

peritoneum in vicinity of cardinal veins, paired gonad primordia

prominent.

260 CHRONOLOGICAL SUMMARY OF ORGAN ANLAGEN

15 mm. total body length stage:

Notochordal cartilage sheath develops, vertebrae begin to appear,

saccule gives rise to lagena (cochlea) and to a basilar chamber, sen-

sory patches appear in ear epithelium, lateral line organ fully devel

oped, mesonephric tubules associated with median cardinal vein.

Metamorphosis at 75 to 90 days:

Loss of horny jaws, widening of mouth, shortening of gut and

changing of its histology to conform with a change from vegetable

diet to an omnivorous diet, loss of tail with its 32 pairs of spinal

nerves and development of two pairs of legs, shedding of larval skin,

disappearance of lateral line system as frog leaves aquatic environ-

ment, loss of caudal veins, full development of tongue, active func-

tioning of endocrine glands (particularly thyroid and pituitary), and

appearance of differentiated gonads containing maturing gametes.

The finished product of normal frog embryology. (Copyright by General

Biological Supply House, Chicago.)

Glossary or Emtryoloîcal Terms

Acidophil—oxyphil: cell constituents which stain with acid dyes, often used

to designate an entire cell type. (See Basophil.)

Acrosome—apical organ at tip of mature spermatozoon, derived from

spermatosphere (idiosome or centrosome) and presumably functional

in aiding penetration of egg cortex by spermatozoon during fertiliza-

tion. (Syn., perforatorium.)

Activation—process of initiating development in egg, normally achieved by

spermatozoon of same species but also accomplished artificially

(parthenogenesis); term also used to refer to stimulation of sper-

matozoon to accelerated activity by chemical (fertilizin) means.

Adnexa—extra-embryonic structures (e.g., yolk sac) discarded before

adult condition is attained.

Aestivation—reduced activity of some animals during heat of summer.

Opposed to hibernation.

Agglutination—cluster formation; a spontaneously reversible reaction of

spermatozoa to certain chemical situations (e.g., egg water).

Aggregation—coming together of cells, such as spermatozoa, without stick-

ing; a non-reversible response comparable to chemotropism.

Albuginea of Testis—stroma of primitive testis which forms a layer between

germinal epithelium and seminiferous tubules.

Albumen—protein substance secreted by walls of oviducts around egg of

reptiles and birds.

Albumen Sac—2-layered ectodermal sac enclosing albumen of chick egg

during early development of embryo, separated for a time from yolk

by vitelline membrane; later to release some of its contents into amni-

otic cavity through ruptured sero-amniotic connection.

Amitosis—direct nuclear division without chromosomal rearrangements;

generally thought to be a sign of decadence or of high specialization,

if it occurs at all.

*A supplementary list of some 350 specialized terms may be found in the author's

"Experimental Embryology, a Manual of Techniques and Procedures," Minneapolis,

Minn., Burgess Publishing Co., 1948.

261

262 GLOSSARY OF EMBRYOLOGICAL TERMS

Amphiblastula—double-structured blastula as in Porifera (sponges).

Amphimixis—mixing of germinal substances accomplished during fertiliza-

tion.

Amphitene—one end of chromosome is thick, one end is thin, moving

toward full pachytene during maturation.

Amplexus—sexual embrace of female amphibian by male, a process which

may (frogs and toads) or may not (urodeles) occur at time of

oviposition.

Anal Plate—thickening and invagination of mid-ventral ectoderm which

meets evaginating endoderm of hindgut, later to be perforated as

proctodeum (anus). (Syn., cloacal membrane.)

Analogy—similarity of parts in respect to function rather than to structure.

Anamniota—forms which never develop an amnion, e.g., cyclostomes,

fishes, amphibia.

Anaphase—phase of mitosis when paired chromosomes are separating at

equatorial plate and begin to move toward ends of spindle.

Anastomosis—joining together, as of blood vessels and nerves, generally

forming a network.

Androgen—hormonal secretion of interstitial tissue of testis.

Androgenesis—development of an egg with paternal (sperm) chromosomes

only, accomplished by removing or destroying egg nucleus before

syngamy.

Angioblast—migratory mesenchyme cell associated with formation of

vascular endothelium.

Animal Pole—region of egg where polar bodies are formed; region of

telolecithal egg containing nucleus and bulk of cytoplasm; gives rise

largely to ectodermal derivatives. (Syn., apical pole or hemispheres.)

Anlage—rudiment; group of cells which indicate a prospective develop-

ment into a part or an organ. (Syn., ebauche or primordium.)

Anterior—toward head; head end. (Syn., cephalic, cranial, rostral.)

Anura—tailless amphibia (e.g., frogs and toads). (Syn., Salientia.)

Aortic Arch—blood vessel which connects dorsal and ventral aortae by

way of visceral arch.

Aqueduct of Sylvius—ventricle of mesencephalon (mesocoel) becomes

aqueduct of Sylvius, connecting with cavities of optic lobes. (Syn.,

iter.)

GLOSSARY OF EMBRYOLOGICAL TERMS 263

Aqueous Humor—fluid which fills anterior and posterior chambers of eye

between lens, probably derived from mesoderm.

Archencephalon—pre-chordal brain, e.g., forebrain. Brain anterior to an-

terior end of notochord.

Archenteric Pouch—See Enterocoel.

Archenteron—primitive gut found in gastrula and communicating with out-

side by blastopore; precursor of embryonic gut. (Syn., gastrocoel,

enteron.)

Arcualia—small blocks of sclerotomal connective tissue involved in forma-

tion of vertebrae.

Asexual Reproduction—reproduction without union of gametes; generally

with no maturation divisions.

Aster—"star-shaped structure" surrounding centrosome (Fol, 1877); lines

radiating in all directions from centrosome during mitosis.

Astral Rays—lines which make up aster.

Astrocytes—stellate-shaped cells arising from spongioblasts of mantle layer,

classified under the more general term of neuroglia.

Atrium—two upper chambers of frog's embryonic heart, later to be known

as auricles.

Attachment Point—point of chromosome to which spindle fiber is attached

and therefore portion of chromosome nearest centrosome in anaphase.

(Syn., centrosome, chromocenter, kinetochore.)

Attraction Sphere—See Centrosphere.

Auricles—two upper chambers of adult frog's heart, derived from em-

bryonic atria.

Autogamy—self-fertilization.

Autosome—any chromosome except so-called sex (X or Y) chromosomes.

Auxocyte—pre-meiotic germ cell. (Syn., primary cyte, meiocyte.)

Axial Filament—central fiber in tail of a spermatozoon.

Axial Mesoderm—that portion of epimeric mesoderm nearest notochord.

(Syn., vertebral plate.)

Axis—imaginary central or median line, generally correlated with a gradient.

Axis of the Cell—imaginary line passing through centrosome and nucleus

of a cell, generally also through geometrical center of cell. In an egg

such an axis generally is also gradient axis of materials such as cyto-

plasm, yolk, pigment, etc.


Axis of the Embryo—imaginary line representing antero-posterior axis of

the future embryo.

Balancers—cylindrical and paired projections of ectoderm with mesen-

chymatous cores, used as adhesive organs in place of (anuran) suckers

by many urodele amphibia.

Balfour's Law—"The velocity of segmentation in any part of the ovumis, roughly speaking, proportional to the concentration of the proto-

plasm there; and the size of the segments is inversely proportional to

the concentration of the protoplasm." The intervals between cleavages

increase in proportion to the amount of yolk which a cell contains in

its protoplasm.

Basal Plate—ventro-lateral wall of myelencephalon, separated from dorso-

lateral alar plate by sulcus limitans.

Basophil—cell constituents having an affinity for basic dyes, often used as

an adjective for an entire cell. (See Acidophil.)

Bidder's Organ—anterior portion of anuran pro-gonad, somewhat ovarian

in character, developing from part of gonad rudiment consisting

wholly of cortex; its development indicates failure of medullary sub-

stance to diffuse to anterior extremity of gonad rudiment.

Biogenetic Law—embryos of higher species tend to resemble embryos of

lower species in certain respects but are never like adults of lower

species. Embryonic development is a gradual deviation from the more

general (phylogenetic) to the more specific characters of the in-

dividual species. Not to be confused with recapitulation theory.

Blastema—indifferent group of cells about to be organized into definite

tissue; nev/ly formed cells covering a cut surface, functional in regenera-

tion of tissues.

Blastocoel—cavity of blastula. (Syn., segmentation or subgerminal cavity.)

Blastoderm—living portion of egg from which both embryo and all of its

membranes are derived. The cellular blastodiscs. "Because the embryo

chooses this as its seat and its domicile, contributing much to its con-

figuration out of its own substance, therefore, in the future we shall

call it blastoderm" (Pander, 1817).

Blastomere—cellular unit of developing egg or early embryo, prior to time

of gastrulation. Smaller blastomeres are micromeres; intermediate

ones are mesomeres; larger ones are macromeres, where there is great

disparity in size.


Blastopore—opening of archenteron (gastrocoel) to exterior, occluded by

yoliv plug in amphibian embryos; consisting of a slit-like space between

elevated margin of blastoderm and underlying yolk of chick egg; repre-

sented in amniota as primitive streak.

Blastopore, Dorsal Lip of—region of first involution of cells in amphibian

gastrula; general area of the "organizer"; original gray crescent area;

cells which turn in beneath potential central nervous system (Am-phioxus) and form roof of archenteron. (Syn., germ ring or marginal

zone.)

Blastopore, Ventral Lip of—region of blastopore opposite dorsal lip; region

which gives rise to peristomial mesoderm of frog. (Syn., germ ring.)

Blastula—stage in embryonic development between appearance of distinct

blastomeres and end of cleavage (i.e., beginning of gastrulation); a

stage generally possessing a primary embryonic cavity or blastocoel;

invariably monodermic. (See specific types under specific names.)

Blood Islands—pre-vascular groups of mesodermal cells found in splanch-

nopleure, from which will arise blood vessels and corpuscles.

Bowman's Capsule—double-walled glomerular cup associated with uri-

niferous tubule.

Branchial—having to do with respiration. (Syn., gill.)

Branchial Arch—visceral arches, beginning with third pair, which contain

blood vessels which (phylogenetically) have respiratory function dur-

ing embryonic development. Mesodermal components which support

those blood vessels are branchial arches. (Syn,, gill arch.) (See Visceral

Arches.)

Branchial Artery—blood vessel which actually passes through gills (ex-

ternal or internal) of frog embryo. (Syn., gill artery.)

Branchial Chamber—closed chamber (except for a single spiracular open-

ing on left side) which encloses internal gills of frog embryo. (Syn.,

opercular or gill chamber.)

Branchial Cleft—opening between branchial arches formed by invaginating

head ectoderm and evaginating pharyngeal endoderm (pouch)

through which water passes from pharynx to outside of frog. (Syn.,

gill cleft or slit, some visceral clefts.)

Branchial Groove—ectodermal invagination anterior or posterior to vis-

ceral arch, which joins branchial pouch to form branchial cleft, in

most instances.


Branchiomery—type of serial metamerism involving respiratory structures

exemplified by visceral arches.

Bud—undeveloped branch, generally an anlage of an appendage (e.g., limb

or wing bud).

Budding—reproductive process by which a small secondary part is pro-

duced from parent organism, and which gradually grows to inde-

pendence.

Bulbus Arteriosus—most anterior division of early, tubular, embryonic

heart which leads from ventricle to truncus arteriosus.

Cardinal Veins—anterior, posterior, and sub-cardinal veins; anterior veins

receive blood from head, including first three segmental veins; posterior

veins receive blood from all pairs of trunk segmental veins and from

veins of Wolffian bodies; paired cardinals enlarge and fuse, left half

degenerates, and balance fuses with developing inferior (posterior)

vena cava.

Cell—protoplasmic territory under control of a single nucleus, whether or

not territory is bounded by a discrete membrane. By this definition a

syncytium is made up of many cells with physiological rather than

morphological boundaries.

Cell Lineage—study of origin and fate of specific blastomeres in embryonic

development. (Syn., cytogeny.)

Cell Theory—body of any living organism is composed of structural and

functional units, the primary agents of organization called cells. Each

cell consists of a nucleus and its sphere of influence, including the

cytoplasm, generally circumscribed by a membrane. "Omnis cellula e

cellula" (Virchow).

Central Canal—See Neurocoel.

Centriole—granular core of centrosome.

Centrosome—granule (centriole) and surrounding sphere of rays (cen-

trosphere) which function as kinetic centers in mitosis. Center of aster

which does not disappear when astral rays disappear. Dynamic cen-

ter of mitosis.

Centrosphere—rayed portion of centrosome; structure in spermatid which

gives rise to acrosome. (Syn., spermatosphere, idiosome, attraction

sphere.

)

Cephalic Flexure—ventral bending of embryonic head at level of midbrain

and hindbrain.


Chimera—compound embryo generally derived by grafting major portions

of two embryos, usually of different species; may be derived by ab-

normal chromosome distribution in cleavage after normal fertiliza-

tion.

Choana—openings of olfactory organ into pharynx, internal nares. Some-

times also used in connection with external olfactory opening.

Chondrification—process of forming cartilage, by secretion of a homoge-

neous matrix between the more primitive cells.

Chondrin—chemical substance in cartilage which makes it increasingly

susceptible to basic stains.

Chondrocranium—that portion of skull which is originally cartilaginous.

Chorda Dorsalis—Syn., notochord.

Chorda Mesoderm—region of the late (amphibian) blastula, arising from

gray crescent area, which will give rise to notochord and mesoderm and

will, if transplanted, induce formation of secondary medullary folds.

Choroid Coat—mesenchymatous and sometimes pigmented coat within

sclerotic coat but surrounding pigmented layer of eye in vertebrate

embryos.

Choroid Fissure—inverted groove in optic stalk whose lips later close

around blood vessels and nerves that enter eyeball.

Choroid Knot—thickened region of fused lips of choroid fissure, near

pupil, from which arise cells of iris.

Chromatid—one of the parts of a tetrad (McClung, 1900); really a longi-

tudinal half of a chromosome.

Chromatin—deeply staining substance of nuclear network and chromo-

somes, consisting of nuclein; gives Feulgen reaction and stains with

basic dyes.

Chromatophore—pigment-bearing cell frequently capable of changing size,

shape, and color; cells responsible for superficial color changes in

animals; behavior under control of sympathetic nervous system or

neurohumors.

Chromidia—granules within cytoplasm which stain like chromatin and

which may actually be extruded chromatin granules.

Chromomere—unit of chromosome recognized as a chromatin granule.

Chromonema—slender thread of chromatin which is core of chromosomeduring mitosis.

Chromophil—cells which have an affinity for dyes.


Chromophobe—cells whose constituents are non-stainable; have no affinity

for dyes.

Chromosome—chromatic or deeply staining bodies derived from nuclear

network and containing a matrix and one or more chromonemata dur-

ing process of mitosis; bodies found in all somatic cells of normal

organism in a number characteristic of the species; bearers of gene.

Cleavage—mitotic division of egg resulting in blastomeres. (Syn., segmenta-

tion. )

Cleavage, Accessory—cleavage in peripheral or deeper portions of (chick)

germinal disc caused by supernumerary sperm nuclei following (nor-

mal) polyspermy, sometimes occurring in urodeles.

Cleavage, Asymmetrical—extremely unequal divisions of egg as in Cteno-

phore.

Cleavage, Bilateral—cleavage in which egg substances are distributed sym-

metrically with respect to median plane of future embryo.

Cleavage, Determinate—cleavage in which certain parts of future embryo

may be circumscribed in certain specific (early) blastomeres; cleavage

which produces blastomeres that are not qualitatively equipotential,

i.e., when such blastomeres are isolated they will not give rise to

entire embryos. (Syn., mosaic development.)

Cleavage, Dexiotropic—cleavage resulting in a right-handed production

of daughter blastomere(s), as in spiral cleavage.

Cleavage, Discoidal—See Cleavage, Meroblastic.

Cleavage, Equatorial—cleavage at right angles to egg axis, opposed to

vertical or meridional; often the typical third cleavage plane. (Syn.,

latitudinal or horizontal cleavage.)

Cleavage, Holoblastic—complete division of egg into blastomeres, gen-

erally equal in size although not necessarily so (e.g., Amphioxus).

(Syn., total cleavage.)

Cleavage, Horizontal—See Cleavage, Equatorial.

Cleavage, Indeterminate—cleavage resulting in qualitatively equipotential

blastomeres in early stages of development. When such blastomeres

are isolated from each other they give rise to complete embryos. Op-

posed to mosaic development. (Syn., regulatory development.)

Cleavage, Latitudinal—See Cleavage, Equatorial.

Cleavage Laws—See specific laws under names of Balfour, Hertwig, and

Sachs.


Cleavage, Levotropic—cleavage resulting in left-handed or counterclock-

wise production of daughter blastomere(s) as in some cases of spiral

cleavage.

Cleavage, Meridional—cleavage along egg axis, opposed to equatorial;

generally the first two cleavages on any egg. (Syn., vertical cleavage.)

Cleavage, Meroblastic—cleavage restricted to peripherally located proto-

plasm, as in chick egg. (Syn., discoidal cleavage.)

Cleavage Nucleus—nucleus which controls cleavage. This may be syn-

gamic nucleus of normal fertilization, egg nucleus of parthenogenetic

or gynogenetic eggs, or sperm nucleus of androgenetic eggs.

Cleavage Path—path taken by syngamic nuclei to position awaiting first

division.

Cleavage, Radial—holoblastic cleavage which results in tiers of cells.

Cleavage, Spiral—cleavage at an oblique angle with respect to egg axis so

that resulting blastomeres (generally micromeres) lie in an interlock-

ing fashion within furrows of original blastomeres, due to intrinsic

genetic factors (e.g., Mollusca).

Cleavage, Superficial—cleavage around periphery of centrolecithal eggs.

(Syn., peripheral cleavage.)

Cochlea—portion of original otic vesicle associated with sense of hearing;

supplied by vestibular ganglion of eighth cranial nerve, having to do

with equilibration.

Coeloblastula—spherical ball of blastomeres with a central cavity (e.g.,

Echinoderms).

Coelom—mesodermal cavity from walls of which gonads develop; cavity

subdivided in higher forms into pericardial, pleural, and peritoneal

cavities. (Syn., extra-embryonic body cavity and exocoel.)

Coitus—copulation of male and female, term generally used in connec-

tion with mammals. Comparable situation in amphibia is called

amplexus.

Collecting Tubule—portion of nephric tubule system leading to nephric

duct (Wolffian, etc.); term also used to refer to tubules which con-

duct spermatozoa from seminiferous tubule to vasa efferentia, within

testis.

Colloid—dispersed substance whose particles are not smaller than 1 ând not larger than 100 jx, approximately. Physical state of proto-

plasm.


Columella—bone in tubo-tympanic cavity of frog which aids in auditory

sensations. (Syn., plectrum, malleus.)

Competence—ability of embryonic area to react to stimulus (e.g., evo-

cator).

Concrescence—coming together of previously separate parts (cell areas)

of embryo, generally resulting in a piling up of parts. One of the

corollaries of gastrulation where a bottle-neck of cell movements

occurs at lips of blastopore. Original meaning (His, 1874) referred

to presumed preformed parts of fish germ ring. (See Confluence.)

Cone, Fertilization—conical projection of cytoplasm from surface of egg

to meet spermatozoon which is to invade egg cortex. Cone makes

contact and then draws sperm into egg. Not universally demonstrated

or seen in frog, but seen in starfish (Chambers). (Syn., exudation

cone.)

Cones of Growth—enlarged outgrowth of neuroblast forms axis cylinder

or axon of nerve fiber and is termed cone of growth because growth

processes by which axon increases in length are supposed to be

located there.

Confluence—similar to concrescence except that this term refers specifi-

cally to "flow" of cells (or areas) together, whether or not they are

piled up.

Constriction—gradual closure of blastopore (diametrical reduction of

germ ring) over yolk toward vegetal pole. May be due to stretching

of marginal zone, to pull or tension of dorsal lip, or even to narrow-

ing of marginal zone. (Syn., convergence [Jordan] or Konzentrisches

Urmundschluss [Vogt].)

Convergence, Dorsal—material of marginal zone moves toward dorsal

mid-line as it involutes during gastrulation, resulting in a compensa-

tory ventral divergence. (Syn., confluence [Smith] or dorsal Reffung

[Vogt].)

Copulation Path—second portion of sperm migration path through egg

toward egg nucleus, when there is any deviation from entrance or

penetration path; path of spermatozoon which results in syngamy.

Cords, Medullary—structures which give rise to urogenital connections

and take part in formation of seminiferous tubules, and are derived

from blastema of mesonephric cords.

Cords, Sex—strands of somatic cells and primordial germ cells growing

from cortex toward medulla of gonad primordium. Best seen in early

stages of testes development.


Cornea—transparent head ectoderm plus underlying mesenchyme form a

layer directly over eye of vertebrates, known as cornea.

Corticin—sex-differentiating substances which spread in some amphibia

by blood stream and in other forms by diffusion and act as a hor-

mone. (See Medullarin.)

Cranial—relative to head; "craniad" means toward head. (Syn., rostral,

cephalad.)

Cranial Flexure—bending of forebrain forward with angle of bend occur-

ring transversely at level of midbrain. (See Cephalic Flexure.)

Crescent, Gray—crescentic area between original animal and vegetal pole

regions on surface of frog's egg, gray in color because of migration

of pigment away from area and toward sperm entrance point (Roux,

1888); region of presumptive chorda-mesoderm, future blastopore,

and anus.

Crest, Neural—paired cell masses derived from ectoderm cells along edge

of former neural plate, and wedged into space between dorso-lateral

wall of closed neural tube and integument. Gives rise to spinal gan-

glia, sympathetic ganglia, and chromatophores.

Crest Segment—original neural crest becomes divided into segments from

which develop spinal and possibly cranial ganglia.

Cross-Fertilization—union of gametes produced by different individuals

which, if they are of different species, may produce hybrids.

Crossing Over—mutual exchange of portions of allelomorphic pairs of

chromosomes during process of synapsis in maturation.

Cyclopia—failure of eyes to separate; median fusion of eyes which may

be due to suppression of rostral block of tissue which ordinarily

separates eyes; exaggeration of vegetativization tendencies.

Cyst—tubular portions of testis within which aggregations of germ cells

mature, often (e.g., Rhomaleum) containing cells all in same stage

of maturation.

Cystic Duct—narrow, proximal portion of embryonic bile duct leading

from gallbladder to common bile duct.

Cytasters—asters arising apart from nucleus in cytoplasm.

Cyte—suffix meaning cell (e.g., osteocyte for bone cell, oocyte for egg

cell). (See specific definitions.)

Cytology—study of cells.


Cytolysis—breakdown of cell indicated by dispersal of formed compo-

nents.

Cytoplasm—material of cell exclusive of nucleus; protoplasm apart from

nucleoplasm.

Delamination—separation (of cell layers) by splitting, a process in meso-

derm formation.

Dermal Bones—bony plates which originate in dermis and cover cartilag-

inous skull.

Dermatome—outer unthickened wall of somite which gives rise to dermis.

(Syn., cutis plate.)

Dermis—deeper layers of skin entirely derived from mesoderm (derma-

tome).

Dermocranium—portion of skull which does not go through an intermedi-

ate cartilaginous stage in development. (Syn., membranocranium.)

Determination—process of development indicated when a tissue, whether

treated as an isolated unit or as a transplant, still develops in the

originally predicted manner.

Determination of Sex—mechanism by which realization of sex differences

is achieved, generally thought to be associated with chromosomal

relations.

Deutencephalon—caudal region of brain which later forms mesencephalon

and rhombencephalon.

Deutoplasm—yolk or secondary food substances of egg; non-living.

Development—gradual transformation of dependent differentiation into

self-differentiation; transformation of invisible multiplicity into a

visible mosaic elaboration of components in successive spatial hier-

archies.

Development, Mosaic—"all the single primordia stand side by side, sepa-

rate from each other like the stones of a mosaic work, and develop

independently although in perfect harmony with each other, into the

finished organism" (Spemann, 1938). Some believe there is pre-

localization of embryonic potencies within egg, test for which would

be self-differentiation.

Development, Regulative—type of development requiring organizer or in-

ductor influences since each of the early blastomeres could develop

into whole embryos. Structures are progressively determined through

action of evocators.


Diencephalon—portion of forcbrain posterior to telencephalon, including

second and third neuromeres.

Differentiation—acquisition of specialized features which distinguish areas

from each other; progressive increase in complexity and organization,

visible and invisible; elaboration of diversity through determination

leading to histogenesis; production of morphogenetic heterogeneity;

process of change from a simple to a complex organism. (Syn., dif-

ferenzierung.

)

Differentiation, Axial—variations in density of chemical and often indefin-

able inclusion in direction of one diameter of the egg, called egg axis.

Differentiation, Dependent—all difi[erentiation that is not self-diflferentia-

tion; development of parts of organism under mutual influences,

such influences being activating, limiting, or inhibiting. Inability of

parts of organism to develop independently of other parts.

Differentiation, Self perseverance in a definite course of development of

a part of an embryo, regardless of its altered surroundings (Roux,

1912).

Diocoel—cavity of diencephalon, ultimate third ventricle.

Diploid—normal complement of chromosomes in somatic and primordial

germ cells, twice the haploid number characteristic of mature gam-

etes.

Diplotene—stage in maturation following pachytene when chromosomes

again appear double and do not converge toward centrosome. Some-

times refers to split individual chromosomes.

Discoblastula—disc-shaped blastula found in cases of discoidal (mero-

blastic) cleavage (e.g.. Cephalopoda and chick).

Distal—farther from any point of reference, away from main body mass.

Divergence, Ventral—divergence of material from mid-ventral line, com-

pensatory to process of dorsal convergence in gastrulation (Vogt).

Diverticulum—blind outpocketing of a tubular structure (e.g., liver or

thyroid anlage).

Dominance—parts of a system which have greater growth momentum and

also which gather strength from the rest, such as dorsal lip of blasto-

pore.

Dorsal Mesentery—membrane formed by doubling of peritoneum from

mid-dorsal line of body cavity, which supports intestine.


Dorsal Root Ganglion—aggregation of neuroblasts which are derived

from neural crests and which send their processes into dorsal horns

of spinal cord.

Dorsal Thickening—roof of mesencephalon which gives rise to optic

lobes.

Duct. See ducts under specific names.

Ductus Arteriosus—See Ductus Botalli.

Ductus Botalli—dorsal portion of sixth pair of aortic arches which nor-

mally becomes occluded after birth, remainder of arch giving rise to

pulmonary arteries. (Syn:, ductus arteriosus.)

Ductus Cuvieri—union of all somatic veins which empty directly into

heart, specifically the vein which unites common cardinals and sinus

venosus. Sometimes regarded as synonymous with common cardinal.

Ductus Endolymphaticus—dorsal portion of original otic vesicle which

has lost all connections with epidermis, and which is partially con-

stricted from region which will form semi-circular canals.

Duodenum—portion of embryonic gut associated with outgrowths of pan-

creas and liver (bile) ducts.

Dyads—aggregations of chromosomes consisting of two rather than four

(tetrad) parts, term used to describe condition during maturation

process.

Ecdysis—process of molting a cuticular layer, shedding of epithelium.

Ectoblast—See Epiblast.

Ectoderm—outermost layer of didermic gastrula. (Syn., epiblast.)

Ectoplasm—external layer of protoplasm of egg cell; layer immediately

beneath cell membrane. (Syn., egg cortex.)

Edema—condition in which tissues hold an excess of water, common in

parthenogenetic tadpoles. (Older spelling: oedema.)

Egg, Alecithal—eggs with little or no yolk. Literally means "without yolk."

Egg, Cleidoic—eggs, such as those of reptiles, birds, and oviparous mam-mals, which are covered by a protective shell.

Egg, Ectolecithal—egg having yolk around formative protoplasm. Op-

posed to centrolecithal.

Egg Envelope—material enveloping egg but not necessarily a part of the

egg, such as vitelline membrane, chorion, jelly, albumen.


Egg, Giant—abnormal polyploid condition where chromosome complexes

are multiplied, resulting in giant cells and embryos.

Egg, Homolecithal—egg (e.g., mammal) in which but little yolk is scat-

tered throughout cytoplasm.

Egg, Isolecithal—eggs with homogeneous distribution of yolk; may be iso-

lecithal, alecithal, or homolecithal.

Egg Jelly—mucin covering deposited on amphibian egg as it passes

through oviduct.

Egg, Macrolecithal—egg with large amount of yolk, generally telolecithal.

Egg Membranes—include all egg coverings such as vitelline membrane,

chorion, and tertiary membranes.

Egg, Microlecithal—egg with small amount of yolk. (Syn., meiolecithal

egg, oligolecithal egg.)

Egg Receptor—part of Lillie's scheme picturing parts that go into the

fertilization reaction involving fertilizin. Egg receptor plus ambo-

ceptor plus sperm receptor gives fertilization.

Egg, Telolecithal—egg with large amount of yolk concentrated at one pole.

Egg Water—watery extract of materials diffusing from living eggs, pre-

sumably the "fertilizin" of Lillie. (Syn., egg water extract.)

Ejaculation—forcible emission of mature spermatozoa from body of male.

Ejaculatory Duct—short portion of mesonephric duct (mammal) between

seminal vesicles and urethra.

Emboitement—preformationist theory of Bonnet and others based on

idea that ovary of first female (Eve?) contained the miniatures of all

subsequently existing human beings. (Syn., encasement theory.)

Embryo—any stage in ontogeny of fertilized egg, generally limited to pe-

riod prior to independent food-getting. Stage between second week

and second month of human embryo.

Endocardium—delicate endothelial tissue forming lining of heart.

Endochondral Bone—bone preformed in cartilage. (Syn., cartilage bone.)

Endoderm—innermost layer of didermic gastmla. (Syn., entoderm.)

Endolymphatic Duct—See Ductus Endolymphaticus.

Endolymphatic Sac—See Saccus Endolymphaticus.

Endoplasm—inner medullary substance of (egg) cell which is generally

granular, soft, watery, and less refractive than ectoplasm.


Entelchy—Driesch's theory of an (intangible) agent controlling develop-

ment. (Syn., elan vital.)

Enterocoel—cavity or pouch within mesoderm just formed by evagination

of gut (enteron) endoderm as in Amphioxus. (Syn., gut pouch,

coelomic pouch, archenteric pouch.)

Enteron—definitive gut of embryo, always lined with endoderm.

Ento-mesoderm—refers to portion of invaginating blastoporal lips which

will induce formation of medullary fields in amphibian embryo.

Entrance Cone—temporary depression on surface of egg following en-

trance of spermatozoon.

Entrance Path—See Path, penetration.

Ependymal Cells—narrow zone of non-nervous and ciliated cells which

surround central canal (neurocoel), from outer ends of which

branching processes extend to periphery, such processes forming a

framework for other cellular elements in spinal cord and brain.

Epiblast—outermost layer of early embryo from which the various germ

layers may be derived.

Epiboly—growing, spreading, or flowing over; process by which rapidly

dividing animal pole cells or micromeres grow over and enclose

vegetal pole material. Increase in areal extent of ectoderm.

Epibranchial Placode—placode (thickening) external to gills related to

lateral line organs and tenth cranial nerves, (Syn., suprabranchial

placode.)

Epidermis—ectodermal portion of skin including cutaneous glands, hair,

feathers, nails, hoofs, and some types of horns and scales.

Epigenesis—development of systems starting with primitive, homogene-

ous, lowly organized condition and achieving great diversification.

Epimere—most dorsal mesoderm, that lying on either side of nerve and

notochord, which gives rise to somites. (Syn., axial mesoderm.)

Epiphysis—evagination of anterior diencephalon of vertebrates which be-

comes separated from brain as pineal (endocrine) gland of adult.

Epithelioid Bodies—endodermal masses arising from second and third

visceral pouches of amphibia.

Epithelium—thin covering layer of cells; may be ectodermal, endodermal,

or mesodermal.


Equational Maturation Division—maturational divisions in which there is

no (qualitative) reduction in chromosomal complex, similar in results

to mitosis.

Equatorial Plate—lateral view of chromosomes, lined up on mitotic spin-

dle, prior to any anaphase movement.

Eustachian Tube—vestige of endodermal portion of hyomandibular pouch

connecting middle ear and pharyngeal cavities and lined with endo-

derm.

Evagination—growth from any surface outward.

"Ex Ovo Omnia"—-all life comes from the egg (Harvey, 1657).

Exogastrula—gastrulation modified experimentally by abnormal condi-

tions so that invagination is partially or totally hindered and there

remains some mesendoderm not enclosed by ectoderm.

Experimental Method—concerted, organized, and scientific analysis of the

causes, forces, and factors operating in any (embryological) system.

External Gills—outgrowths of (amphibian) branchial arches which func-

tion as temporary (anura) or permanent neotonic (urodela) respira-

tory organs.

Extra-Embryonic—refers to structures apart from embryonic body, such

as membranes.

Fate Map—map of blastula or early gastrula stage which indicates pro-

spective significance of various surface areas, based upon previously

established studies of normal development aided by means of vital

dye markings.

Fate, Prospective—destination toward which we know, from previous ex-

perience, that a given part would develop under normal conditions;

lineage of each part of egg through its cell descendants into a definite

region or portion of adult organism.

Fertilization—activation of egg by sperm and syngamy of pronuclei; union

of male and female gamete nuclei.

Fertilization, Anti "eggs contain within their interior a substance capa-

ble of combining with the agglutinating group of the fertilizin, but

which is separate from it as long as the egg is inactive" (Lillie).

Feulgen Reaction—chemical test for thymo-nucleic acid, used as a specific

staining test for chromatin.

Field—mosaic of spatio-temporal activities within developing organism.


Field, Morphogenetic—embryonic field out of which will normally develop

certain specific structures.

Flexure—refers to a bending such as cranial, cervical, and pontine flexures.

Also dorsal and lumbo-sacral flexures of the pig.

Follicle—cellular sac within which egg generally goes through early matu-

ration stages.

Forebrain—most anterior of first three primary brain vesicles, associated

with lateral opticoels. (Syn., prosencephalon.)

Foregut—more anterior portion of enteric canal, first to appear, aided by

development of pharyngeal derivatives.

Fovea Germinativa—pigment-free spot of animal hemisphere where am-

phibian germinal vesicle gives off its polar bodies.

Frontal—plane at right angles to both transverse and sagittal, dividing dor-

sal from ventral. (Syn., coronal.)

Gamete—differentiated (matured) germ cell, capable of functioning in

fertilization (e.g., sperm or egg cell, germ cell).

Gametogenesis—process of developing and maturing germ cells.

Ganglion—aggregation of neurons, generally derived from a neural crest

(e.g., cranial and spinal ganglia).

Ganglion, Acoustic—eighth cranial ganglion from which fibers of eighth

cranial nerve arise, purely sensory.

Ganglion, Acustico-facialis—early undifferentiated association of seventh

and eighth cranial ganglia.

Ganglion, Gasserian—fifth cranial ganglion, carrying both sensory and

motor fibers. (Syn., trigeminal ganglion, semilunar ganglion.)

Ganglion, Geniculate—ganglion at root of facial (VII) cranial nerve,

carrying both sensory and motor fibers.

Ganglion, Nodosal—ganglion associated with vagus (X) cranial nerve

which carries afferent fibers to pharynx, larynx, trachea, oesophagus,

and thoracic and abdominal viscera.

Ganglion, Petrosal—ganglion associated with glossopharyngeal (IX) cra-

nial nerve, more peripheral than superior ganglion carrying sensory

fibers from pharynx and root of tongue.

Ganglion, Superior—ganglion associated with glossopharyngeal (X) cra-

nial nerve, mesial to petrosal ganglion.


Gasserian Ganglion—fifth cranial or trigeminal ganglion, derived from

midbrain.

Gastraea Theory—theory of Haeckel that since all higher forms have gas-

trula stages there may have existed a common ancestor built on the

plan of a permanent gastrula, as are the recent Coeloenterata.

Gastral Mesoderm—mesoderm derived from dorso-lateral bands (entero-

coelic) in Amphioxus or from dorsal lip in frog. Opposed to peristo-

mial mesoderm.

Gastrocoel—cavity formed during process of gastrulation. (Syn., archen-

teron.)

Gastrula—didermic embryo, possessing a newly formed cavity, gastrocoel

or archenteron. The two layers are ectoderm and endoderm.

Gastrular Cleavage—separation of ectoderm and endoderm, during gas-

trulation, by a slit-like crevice, actually compressed blastocoel.

Gastrulation—dynamic process involving cell movements which change

embryo from a monodermic to either a di- or tridermic form. Gen-

erally involves inward movement of cells to form enteric endoderm.

Description includes epiboly, concrescence, confluence, involution,

invagination, extension, and convergence.

Genital—refers to reproductive organs or processes, or both.

Genital Ducts—any ducts which convey gametes from their point of origin

to region of insemination (e.g., collecting tubules, vas deferens, vas

efTerens, seminal vesicle, oviduct, uterus, etc.).

Genital Ridge—initial elevation or thickening for development of external

genitalia.

Germ—egg throughout its development, or at any stage.

Germ Cell—cell capable of sharing in reproductive process, in contrast

with a somatic cell (e.g., sperm or egg cell). (Syn., gamete.)

Germ Layer—more or less artificial spatial and histogenic distinction of

cell groups beginning in gastrula stage, consisting of ectodermal,

endodermal, and mesodermal layers. No permanent and clear-cut dis-

tinction, as shown by transplantation experiments.

Germ Plasm—hereditary material, generally referring specifically to the

genotype. Opposed to somatoplasm.

Germ Ring—ring of cells showing accelerated mitotic activity, generally

a synonym for lips of blastopore. The rapidly advancing cells in epib-

oly.


Germinal Epithelium—peritoneal epithelium out of which reproductive

cells of both male and female presumably develop. (Syn., germinal

ridges, gonadal ridges.)

Germinal Localization—every area of blastoderm or of unfertilized egg,

corresponds to some future organ. Unequal growth produces differ-

entiation of parts (His, 1874). This concept led to Mosaic Theory of

Roux (see Fate Map, p. 101).

Germinal Spot—nucleolus of ovum.

Germinal Vesicle—pre-maturation nucleus of egg.

Gestalten—system of configuration consisting of a ladder of levels; elec-

trons, atom, molecule, cell tissue, organ, and organism, each one of

which exhibits specifically new modes of action that cannot be under-

stood as mere additive phenomena of the previous levels. With each

higher level new concepts become necessary. The parts of the cell

cannot exist independently, hence the cell is more than a mere aggre-

gation of its parts—it is a patterned whole. A coherent unit reaching

a final configuration in space (W. Kohler). Gestaltung means forma-

tion.

Gill—See Branchial Arch, Branchial Chamber, Branchial Cleft.

Gill Plate—elevated and thickened areas of ectoderm posterior to sense

plate of embryo where visceral grooves will subsequently form.

Gill Rakers—ectodermal, finger-like obstructions which sift water as it

passes from oral cavity to gill chambers of frog tadpole.

Glia Cells—small rounded supporting cells of spinal cord, derived from

germinal cells of neural ectoderm.

Glomerulus—aggregation of capillaries associated with branches of dorsal

aorta but lying within substance of functional kidney; function is

excretory.

Glomus—vascular aggregations within head kidney or pronephros, never

to become a glomerulus.

Glottis—opening between pharynx and larynx.

Gonad—organ within which germ cells are produced and generally matured

(e.g., ovary or testis). (Syn., sex or germ gland.)

Gonadromorph—condition in which part of an animal may be male and

another part female; not to be confused with hermaphroditism.

Gonium—suffix referring to a stage in maturation of a germ cell prior to

any maturation division (e.g., spermatogonium, or oogonium).


Gonoduct—See Genital Ducts.

Gradient—gradual variation of developmental forces along an axis; scaled

regions of preference. (See Axis.)

Gray Crescent—See Crescent, Gray.

Growth—developmental increase in total mass of protoplasm at expense of

raw materials; an embryonic process, generally differentiation; cell

proliferation.

Gynogenesis—development of sperm activated egg but without benefit of

sperm nucleus.

Haploid—having a single set of chromosomes not appearing in allelo-

morphic pairs, as in mature gametes. Opposed to diploid, or the con-

dition in somatic cells.

Harmonious-Equipotential System—embryonic system in which all parts

are equally ready to respond to organism as a whole. Isolated blas-

tomeres of such a system may give rise to complete embryos.

Hatching—beginning of larval life of amphibian, accomplished by tem-

porarily secreted hatching enzymes which aid embryo to escape from

its gelatinous capsule.

Hepatic Sinusoids—maze of dilated and irregular capillaries between loosely

packed framework of hepatic tubules.

Hepatic Veins—veins from liver to heart, originating as anterior portions

of vitelline veins of amphibia.

Hepatic Veins, Portal—remnants of posterior portions of left vitelline vein.

Hermaphrodite—individual capable of producing both spermatozoa and

ova.

Hermaphrodite, Protandrous—male elements mature prior to female ele-

ments in hermaphrodite.

Hermaphrodite, Protogynous—female elements mature prior to male ele-

ments in hermaphrodite.

Hertwig's Law—nucleus tends to place itself in center of its sphere of ac-

tivity; longitudinal axis of mitotic spindle tends to lie in longitudinal

axis of yolk-free cytoplasm of cell.

Heteroagglutinin—agglutinin (fertilizin) of eggs which acts on sperm of

different species, substance extractable from egg water which causes

irreversible agglutination of foreign species.


Heterozygous—condition in which zygote is composed of gametes bearing

allelomorphic genes. Opposed to homozygous.

Hibernation—spending the cold (winter) period in a state of reduced ac-

tivity.

Hindbrain—most posterior of the three original brain divisions. (Syn.,

rhombencephalon.

)

Hindgut—portion of amphibian embryonic gut just anterior to neurenteric

canal. Level of origin of rectum, cloaca, post-anal gut, and caudal

portions of urogenital systems.

Histogenesis—development of tissues.

Homoiothermal—pertaining to a condition in which temperature of body

of organism is under control of an internal mechanism; body tem-

perature regulated. Opposed to poikilothermal.

Homology—similarity in structure based upon similar embryonic origin.

Homoplastic—pertaining to a graft to an organism of same species, or even

to another position on the same individual. (Syn., autoplastic.)

Homozygous—condition in which zygote is composed of gametes bearing

identical rather than allelomorphic genes.

Horizontal—unsatisfactory term sometimes used synonymously with frontal,

longitudinal, and even sagittal plane or section. Actually means across

the lines of gravitational forces.

Hormone—secretion of a ductless (endocrine) gland which can stimu-

late or inhibit activity of distant parts of biological system already

formed.

Hyaloplasm—viscid liquid regarded as essential living protoplasm.

Hybrid—successful cross between different species (e.g., horse and ass

give a mule, which is sterile).

Hyoid Arch—mesodermal mass between hyomandibular and first branchial

clefts, or between first and second visceral pouches or clefts which give

rise to columella and parts of hyoid apparatus. (Syn., second visceral

arch.

)

Hyomandibular—pertaining to pouch, cleft, or slit between mandibular and

hyoid arches.

Hyperplasia—overgrowth; abnormal or unusual increase in elements com-

posing a part.

Hypertrophy—increase in size due to increase in demands upon part con-

cerned.


Hypochordal Rod—transitory string of cells constricted off between dor-

sal wall of midgut and notochord of amphibian embryo, between level

of pancreas and tail, and disappearing before hatching time. (Syn.,

sLib-notochordal rod.)

Hypomere—most ventral segment of mesoderm out of which develop

somatopleure, splanchnopleure, and coelom. (Syn., lateral plate

mesoderm.)

Hypophysis—ectodermally derived solid structure arising anterior to

stomodeum and growing inwardly toward infundibulum to give rise

to anterior and intermediate parts of pituitary gland.

Hypoplasia—undergrowth or deficiency in elements composing a part.

Hypothesis—complemental supposition; presumption based on fragmentary

but suggestive data offered to bridge a gap in incomplete knowledge

of the facts. May be offered as an explanation of facts unproved, un-

til subjected to verification or disproof.

Idiosome—material out of which acrosome is formed during metamorphosis

of spermatid to spermatozoon. (Syn., spermatosphere, centrosphere.

)

Induction—successive and purposeful influences which bring about

morphogenetic changes within embryo.

Inductor—a loose word which includes both organizer and evocator (Need-

ham). Generally means a piece of living tissue which brings about

differentiation within otherwise indifferent tissue.

Infundibulum of the Brain—funnel-like evagination of floor of diencephalon

which, along with hypophysis, will give rise to pituitary gland of adult.

Infundibulum of the Oviduct—See Ostium Abdominale Tubae.

Ingression—inward movement of yolk endoderm of amphibian blastula

(Nicholas, 1945).

Insemination—process of impregnation; fertilization.

Interauricular Septum—longitudinal sheet of mesodermal tissue which

grows ventrally from roof of atrial chamber to divide it into right and

left halves.

Interkinesis—resting stage between mitotic divisions.

Intermediate Cell Mass—narrow strip of mesoderm which, for a time, joins

dorsal epimere with ventral hypomere, being made up of a dorsal

portion continuous with dorsal wall of somite and somatic meso-

derm and a ventral portion continuous with ventral wall of somite and


splanchnic mesoderm. Source of origin of excretory system. (Syn.,

nephrotome or middle plate.)

Internal Gills—filamentous outgrowths on posterior side of first three pairs

of branchial arches and a single row on anterior side of fourth pair

of branchial arches of frog tadpole, which have a respiratory function

concurrent with and following absorption of external gills.

Internal Limiting Membrane—membrane which develops on innermost

surface of inner wall of optic cup during fourth day of chick develop-

ment.

Intersex—individual without typical sexual differentiation.

Interstitial Cells—specialized cells between seminiferous tubules of testis

which produce hormones.

Interstitial Tissue of Testis—cell aggregations between seminiferous tubules

of testis which elaborate a male sex hormone.

Invagination—folding or inpushing of a layer of cells into a preformed

cavity, as in one of the processes of gastrulation. Opposed to involu-

tion.

Involution—rolling inward or turning in of cells over a rim, as in gastrula-

tion of chick embryo.

Iris—^narrow zone bounding pupil of eye in which two layers of optic cup

become blended so that pigment from outer layer invades material of

inner layer, giving eye a specific color by variable reflection.

Isogamy—similar gametes, without differentiations into spermatozoa and

ova.

Isolation Culture—removal of a part of an organism and its maintenance

in a suitable medium in living condition.

Isthmus of the Brain—depression in dorsal wall of embryonic brain which

partially separates mesencephalon from metencephalon.

Isthmus of the Oviduct—short, tubular, posterior end of oviduct (e.g.,

chick) in which fluid albumen and shell membranes are applied to egg.

Iter—See Aqueduct of Sylvius.

Jacobson's Organ—ventro-medial evaginations from olfactory pits (am-

phibia and reptilia) which later become glandular and sensitive olfac-

tory epithelia.

Jelly—mucin covering of amphibian egg derived from oviduct and applied

outside vitelline membrane.


Jugular Veins—veins which bring blood from head, superior or internal

jugular being anterior cardinal veins and inferior jugular veins growing

toward lower jaw and mouth from base of each ductus Cuvieri.

Karyoplasm—protoplasm within confines of nucleus.

Kern-Plasma Relation—ratio of amount of nuclear and of cytoplasmic

materials present in the cell. It seems to be a function of cleavage to

restore kern-plasma relation from unbalanced condition of ovum with

its excessive yolk and cytoplasm to new ratio of gastrula or somatic

cell.

Lamina Terminalis—point of suture of anterior neural folds (i.e., anterior

neuropore) where they are finally separated from head ectoderm; it

consists of a median ventral thickening at anterior limit of telen-

cephalon (from anterior side of optic recess to beginning of velum

transversum) and includes anterior commissure of torus transversus.

Larva—stage in development when organism has emerged from its mem-branes and is able to lead an independent existence, but may not have

completed its development. Generally (except in cases of neoteny or

paedogenesis) larvae cannot reproduce.

Larynx—anterior part of original laryngo-tracheal groove which becomes

a tube opening into pharynx by way of glottis.

Lateral—either right (dextral) or left (sinistral) side; laterad means toward

the side.

Lateral Line Organs (or System)—line of sensory structures along side

of body of fishes and amphibia, generally embedded in skin and in-

nervated by a branch from vagus ganglion, presumably concerned

with recognition of low vibrations in water. Appears first at about 4

mm. stage in frog embryo. (Syn., ramus lateralis.)

Lateral Mesocardium—septum posterior to heart extending from base of

each vitelline vein obliquely upward to dorso-lateral body wall, repre-

senting one of the three parts of septum transversum.

Lateral Mesoderm—See Lateral Plate Mesoderm.

Lateral Neural Folds—See Neural Fold.

Lateral Plates or Lateral Plate Mesoderm—lateral mesoblast within which

body cavity (coelom and exocoel) arises. (Syn., lateral mesoderm.)

Lateral Ventricles of the Brain—thick-walled and laterally compressed

cavities of prosencephalon which open into third ventricle by wayof foramen of Monro; walls will become cerebral hemispheres.


Lecithin—fat from an. animal organism which is phosphorized in form of

phosphatides.

Lens—thickening in head ectoderm opposite optic cup at about time of

hatching in frog embryo; it becomes a placode, invaginates to acquire

a vesicle, and then pinches off into space of optic cup as a lens. Inner

surface convex; substance fibrous.

Lens Placode—early thickened ectodermal primordium of lens.

Leptotene—stage in maturation which follows last -gonial division and

prior to synaptene stage, structurally similar to resting cell stage. The

chromatin material in form of a spireme. Term means thin, diffuse.

Lipids—fats and fatty substances such as oil and yolk (lecithin) found in

eggs (e.g., cholesterol, ergosterol).

Lips of the Blastopore—See Blastopore, Lips of.

Localization—cytological separation of parts of the mosaic egg, each of

which has a known specific subsequent differentiation. There is often

a substratum associated with these areas, made up of pigmented

granules, but it is cytoplasm rather than pigmented elements in which

localization occurs.

Macromere—^larger of the blastomeres where there is a conspicuous size

difference.

Malpighian Body—unit of functional kidney including Bowman's capsule

and glomerulus. (Syn., renal corpuscle, Malpighian corpuscle.)

Mandibular Arch—rudiment of lower jaw, mesodermal, and anterior to

first or hyomandibular pouch.

Mantle Fibers—those fibers of mitotic spindle which attach chromosomes

to centrosomes.

Mantle Layer of the Cord—zone of developing spinal cord with densely

packed nuclei slightly peripheral to germinal cells from which they

are derived. Includes elongated cells of ependyma.

Maturation—process of transformation of a primordial germ cell (sper-

matogonium or oogonium) into a functionally mature germ cell, the

process involving two special divisions, one of which is always meiotic.

Divisions known as equational and reductional.

Mechanism—assumption that biological processes do not violate physical

and chemical laws but that they are more than the mere function-

ing of a machine because material taken into the organism becomes an


integral part of the organism, through chemical changes. (Syn., the

scientific attitude.) (See Vitalism.)

Meckel's Cartilage—core of lower jaw derived from ventral part of car-

tilaginous mandibular arch.

Median plane—"middle" plane, as of an embryo. May be median sagittal

or median frontal.

Medulla Oblongata—that portion of adult brain derived from rhom-

bencephalon.

Medullarin—sex differentiating substance spread in some amphibia by

blood stream as a hormone and in other forms by diffusion. (See

Corticin.)

Medullary—See terms under Neural, such as canal, fold, groove, plate,

tube.

Medullary Cords—that portion of suprarenal glands derived from sym-

pathetic nervous system; central cords. Also that portion of embryonic

gonad presumably derived from pre-migratory germ cells upon reach-

ing genital ridge.

Meiosis—process of nuclear division found in maturation of germ cells,

involving a separation of members of pairs of chromosomes. (Syn.,

reductional division.)

Melanophore—well with black or brown pigment (melanin), derived from

neural crests and migrating throughout body.

Membrane Bone—bone developed in regions occupied by connective tissue,

not cartilage.

Membrane, Vitelline—See Vitelline Membrane.

Membranes—See Egg Membranes.

Meroblastic Cleavage or Ova—See under Cleavage or Egg.

Mesencephalon—section of primary brain between posterior level of

prosencephalon and an imaginary line drawn from tuberculum pos-

terius to a point just posterior to dorsal thickening. Gives rise to optic

lobes, crura cerebri, and aqueduct of Sylvius. (Syn., midbrain.)

Mesenchyme—form of embryonic mesoderm or mesoblast in which

migrating cells unite secondarily to form a syncitium or network hav-

ing nuclei in thickened nodes between intercellular spaces filled with

fluid; often derived from mesothelium.


Mesendoderm—newly formed layer of (urodele) gastmla before there has

been any separation of endoderm and mesoderm. (Syn., mesentoderm,

mesentoblast, ento-mesoblast.)

Mesentery—sheet of (mesodermal) tissue generally supporting organ sys-

tems (e.g., mesorchium, mesocardium).

Mesial—median, medial, middle.

Mesoblast, Gastral—See Gastral Mesoderm.

Mesoblast, Peristomial—involuted, ventral lip mesoderm, continuous with

gastral mesoderm from dorsal lip.

Mesocardium—mesentery of heart; may be dorsal, ventral, or lateral. (See

under Lateral Mesocardium.)

Mesoderm—the third primary germ layer developed in point of time, maybe derived from endoderm in some forms and from ectoderm in

others. (See other terms such as Mesoblast, Mesenchyme, Lateral

Plate Mesoderm, Epimere, Mesomere, Hypomere, Gastral Meso-derm, Peristomial Mesoderm, Axial Mesoderm, etc.)

Mesomere—cell of intermediate size where there are conspicuous size dif-

ferences in an early embryo; also refers to intermediate cell mass:

intermediate mesoderm.

Mesonephric Duct—duct which grows posteriorly from mesonephros to

cloaca and functions also as vas deferens in male. (Syn., Wolffian

duct.)

Mesonephric Tubules—primary, secondary, and sometimes tertiary tubules

developing in Wolffian body, functioning in adult amphibia.

Mesonephros—Wolffian body, or intermediate kidney, functional as kidney

in adult fish and amphibian.

Mesorchium—mesentery (mesodermal) which surrounds and supports testis

to body wall.

Mesothelium—epithelial layers or membranes of mesodermal origin.

Mesovarium—mesentery (mesodermal) which suspends ovary from dorsal

body wall.

Metamerism—serial segmentation, as seen in nervous, muscular, and circu-

latory systems.

Metamorphosis—end of larval period of amphibia when growth is sus-

pended temporarily. There is autolysis and resorption of old tissues

and organs such as gills, and development of new structures such

as eyelids and limbs; changes in structure correlated with changes in


habitat from one that is aquatic to one that is terrestrial; change in

structure without retention of original form, as in change from

spermatid to spermatozoon.

Metaphase—stage in mitosis when paired chromosomes are lined up on

equatorial plate midway between amphiasters, supported by mitotic

spindle, prior to any anaphase movement.

Micromere—smaller of cells when there is a conspicuous difference in size,

characteristic of Annelids and Molluscs.

Micropyle—aperture in egg covering through which spermatozoa mayenter; in such eggs the only possible point of insemination (e.g., manyfish eggs).

Midbrain—See Mesencephalon.

Midgut—that portion of archenteron which will give rise to intestines.

Milieu—Term used to include all of the physico-chemical and biological

factors surrounding a living system (e.g., external or internal milieu).

Mitochondria—small, permanent, cytoplasmic granules which stain with

Janus green B and Janus red; granules which have powers of growth

and division; probably lipoid.

Mitosis—cytoplasmic division involving a nucleus and spindle apparatus.

Mitotic Index—proportion in any tissue and at any specified time of the

dividing cells; percentage of actively dividing cells.

Monospermy—fertilization accomplished by only one sperm. Opposed to

polyspermy.

Monro, Foramina of—tubular connections between single third and paired

lateral ventricles of forebrain.

Morphogenesis—all of the topogenetic processes which result in structure

formation; origin of characteristic structure (form) in an organ or in

an organism compounded of organs.

Morphogenetic Movements—cell or cell area movements concerned with

formation of germ layers (e.g., during gastrulation) or of organ

primordia.

Morula—spherical mass of cells, as yet without segmentation cavity.

Mosaic—type of egg or development in which fate of all parts is fixed at

an early stage, possibly even at time of fertilization. Local injury or

excisions generally result in loss of specific organs in developing em-

bryo. Opposed to regulative development.

Miillerian Duct—See Oviducts.


Muscle Plate—See Myotome.

Myeloblasts—muscle-forming (embryonic) cells.

Myoblasts—formative cells within myotome or muscle plate which will

give rise to true striated muscles of adult.

Myocardium—muscular part of heart arising from splanchnic mesoblast.

Myocoel—cavity within which ovaries of Amphioxus develop; temporary

cavities within myotomes which may have been connected with coelom.

Myotome—thickened primordium of muscle found in each somite, (Syn.,

muscle plate.)

Nares, External—external openings of tubes which are connected with

olfactory vesicles.

Nares, Internal—openings of tubular organ from olfactory placodes into

anterior part of pharynx of 12 mm. frog tadpole. (Syn., choanae.)

Nasal Choanae—openings of olfactory chambers into mouth.

Nasal Pit—See Olfactory Pit.

Nebenkern—cytological structure near nucleus of early spermatid.

Neoteny—condition of many urodeles and of experimentally produced

(thyroidless) anuran embryos in which larval period is extended or

retained, i.e., larvae fail to go through normal metamorphosis. Sexual

maturity in larval stage (e.g., axolotl, Necturus).

Nephrocoel—cavity, found in nephrotome or intermediate cell mass, which

temporarily joins myocoel and coelom.

Nephrogenic Cord—continuous band of intermediate mesoderm (meso-

mere) without apparent segmentation, prior to budding off of meso-

nephric tubules.

Nephrogenic Tissue—intermediate cell mass, mesomere, or nephrotome

which will give rise to excretory system.

Nephrostome—funnel-shaped opening of kidney tubules into coelom; outer

tubules of amphibian mesonephric kidney acquire ciliated neph-

rostomal openings from coelom and shift their connections to renal

portal sinus.

Nephrotome—intermediate cell mass.

Nephrotomic Plate—intermediate mesoderm, mesomere.

Nerve, Abducens—sixth (VI) cranial nerve arising from basal plate of

rhombencephalon which controls external rectus muscles of eye.


Nerve, Auditory—eighth ( VllI ) cranial nerve, purely sensory, arising from

acoustic ganghon and associated with geniculate ganglion of seventh

nerve.

Nerve, Facial—seventh (VII) cranial nerve, both sensory and motor, re-

lated to taste buds and facial muscles.

Nerve, Glossopharyngeal—ninth (IX) cranial nerve, mixed, associated

with superior and petrosal ganglia.

Nerve, Oculomotor—third (III) cranial nerve which arises from neuro-

blasts in ventral zone of midbrain near median line just before hatch-

ing in frog tadpole.

Nerve, Vagus—tenth (X) cranial nerve, mixed, arising from rhom-

bencephalon and associated with jugular ganglion.

Nervous Layer—innermost of two layers found in roof of segmentation

cavity of amphibian blastula, from which bulk of central nervous sys-

tem is developed.

Neural Arch—ossified cartilages which extend dorsally from centrum

around nerve cord.

Neural Canal—See Neurocoel and Neural Tube.

Neural Crest—continuous cord of ectodermally derived cells lying on each

side in angle between neural tube and body ectoderm, separated from

ectoderm at time of closure of neural tube and extending from ex-

treme anterior to posterior end of embryo; material out of which

spinal and possibly some cranial ganglia develop, and related to de-

velopment of sympathetic ganglia by cell migration.

Neural Fold—elevation of ectoderm on either side of thickened and de-

pressing medullary plate; folds which close dorsally to form neural

tube. (Syn., medullary folds.)

Neural Groove—depression caused by sinking in of center of medullary

plate to form a longitudinal groove, later to be incorporated within

neural tube (spinal cord). (Syn., medullary groove.)

Neural Plate—thickened broad strip of ectoderm along future dorsal side

of all vertebrate embryos, later to give rise to central nervous system.

(Syn., medullary plate.)

Neural Tube—tube formed by dorsal fusion of neural folds, rudiment of

nerve or spinal cord.

Neurenteric Canal—posterior neurocoel where it is connected with clos-

ing blastopore and posterior enteron of amphibian; the large com-


mon nervous and enteric chamber of Amphioxus; the Kupffer's vesicle

of fish embryo; possibly the primitive pit of chick embryo. (Syn.,

notochordal canal, primitive pit.)

Neuroblasts—primitive or formative nerve cells, probably derived (along

with epithelial and glia cells) from germinal cells of embryonic neural

tube.

Neurocoel—cavity of neural tube, formed simultaneously with closure of

neural folds. (Syn., central canal, neural canal.)

Neurocranium—dorsal portion of skull associated with brain and sense

organs.

Neuroglia—see Glia Cells.

Neuropore—temporary opening into neural canal due to a lag in fusion of

neural folds at anterior extremity; permanent in Amphioxus and in

vicinity of epiphysis of higher vertebrates.

Neurula—stage in embryonic development which follows gastrulation and

during which neural axis is formed and histogenesis proceeds rapidly.

Notochord and neural plate are already differentiated, and basic verte-

brate pattern is indicated,

Notochord—rod of vacuolated cells representing axis of all vertebrates,

found beneath neural tube and dorsal to archenteron. Thought to be

derived from or simultaneously with endoderm.

Notochordal Sheath—double mesodermal sheath around notochord consist-

ing of an outer elastic sheath developed from superficial chorda cells

and an inner secondary or fibrous sheath from chorda epithelium.

Nucleolus—the body generally within the nucleus which has no affinity for

chromatin dyes, but stains with acid or cytoplasmic dyes. Function un-

known. (Syn., plasmosome.)

Oesophagus—elongated portion of foregut between future glottis and

opening of bile duct of frog embryo; temporarily occluded just behind

glottis but opens again.

Olfactory Lobes—anterior extremities of telencephalic cerebral lobes,

partially constricted, associated with first pair of cranial nerves.

Olfactory Pit—depressions within olfactory placodes of 6 mm. frog embryo

which will become olfactory organs (external nares).

Olfactory Placode—thickened ectoderm lateral to stomodeal region found

in 5 mm. frog embryo, primordia of olfactory pits.

"Omne Vivum e Vivo"—all life is derived from preexisting life (Pasteur).


Omnipotent—term used in connection with a cell which could, under

various conditions, assume every cytological differentiation known to

the species or which, by division, could give rise to such varied differ-

entiations.

"Omnis Cellula e Cellula"—all cells from preexisting cells (Virchow).

Ontogeny—developmental history of an organism; sequence of stages in

early development.

Oocyte—presumptive egg cell after initiation of growth phase of maturation.

(Syn., ovocyte.)

Oogenesis—process of maturation of ovum; transformation of oogonium

to mature ovum. (Syn., ovogenesis.)

Oogonia—multiplication (mitotic) stage prior to maturation of presumptive

egg cell (ovum), found most frequently in peripheral germinal epithe-

lium.

Ooplasm—cytoplasmic substances connected with building rather than re-

serve materials utilized in developmental processes.

Opercular Chamber—See Branchial Chamber.

Operculum—integumentary growth posteriorly from each of the hyoid

arches of frog embryo, which covers and encloses gills.

Optic Chiasma—thickening in forebrain ventral to infundibulum, found

as a bunch of optic nerve fibers in future diencephalon.

Optic Cup—invagination of outer wall of primary optic vesicle to form

a secondary optic vesicle made up of two layers; a thick internal or

retinal layer continuous at pupil and choroid fissure, and a thin external

layer which is pigmented. Cavity of cup becomes future posterior

chamber of eye.

Optic Lobes—thickened, evaginated, dorso-lateral walls of mesencephalon.

Optic Recess—depression in forebrain anterior to optic chiasma which

leads to optic stalks.

Optic Stalk—attachment of optic vesicle to forebrain, at first a tubular

connection between optic vesicle and diencephalon. Lumen is later

obliterated by development of optic nerve fibers.

Optic Vesicle—evagination of forebrain ectoderm to form primary optic

vesicles which in turn invaginate to form secondary optic vesicles or

optic cups of eyes.

Opticoel—cavity of primary optic cup.


Oral Plate—stomodeal ectoderm and pharyngeal endoderm fused to form

oral membrane. Breaks through to form mouth. (Syn., pharyngeal

membrane, oral membrane, stomodeal plate.)

Oral Suckers—elongated, pigmented depressions at antero-ventral ends

of mandibular arches of frog embryo which give rise to mucous glands;

with adhesive function.

Organization—indicated by interdependence of parts and the whole. "Whenelements of a certain degree of complexity become organized into an

entity belonging to a higher level of organization," says Waddington,

"we must suppose that the coherence of the higher level depends on

properties which the isolated elements indeed possessed but which

could not be entered into certain relations with one another." See

Gestalten.

Organizer—chorda-mesodermal field of amphibian embryo; a tissue area

which has power of organizing indifferent tissue into a neural axis;

possibly comparable to Henson's node of chick embryo.

Osteoblasts—mesenchymal cells which actively secrete a calcareous ma-terial in formation of bone; bone-forming cells.

Osteoclasts—bone-destroying cells; cells which appear in and tend to

destroy formed bone; constantly active, even in embryo.

Ostium Abdominale Tubae—most anterior, fimbriated end of oviduct in

female vertebrates; point of entrance of ovulated egg into oviduct;

double in amphibia. (Syn., infundibulum of oviduct, tubal ridge.)

Otic Vesicle—auditory vesicle, otocyst.

Otocyst—original auditory vesicle appearing at level of rhombencephalon

in amphibian embryo just before hatching, forming first as a placode.

(Syn., auditory vesicle.)

Oviducal Membranes of Ovum—tertiary membranes applied over egg as

it passes through oviduct.

Oviducts—paired MUllerian ducts in both males and females, which gen-

erally persist in males.

Ovigerous Cords—columns or strands of tissue which divide germinal

epithelium of primordium of ovary, carrying primordial germ cells

with them and later breaking up into nests of cells, each of which

contains an oogonium. (Syn., egg tubes or cords of Pfliiger [mammal].)

Oviposition—process of laying eggs.

Ovocyte—See Oocyte.


Ovogenesis—See Oogenesis.

Ovogonia—See Oogonia.

Ovulation—release of egg from ovary, not necessarily from body.

Ovum—Latin for egg.

Pachytene—stage in maturation when allelomorphic pairs of chromosomes

are fused (telosynapsis or parasynapsis) so as to appear haploid, during

which process crossing over may occur; stage just prior to diplotene.

Term means thick or condensed. (Syn., diplonema.)

Paedogenesis—reproduction during larval stage; precocious sex develop-

ment.

Pancreas—digestive and endocrine glands arising as single posterior and

single anterior primordia in vicinity of liver.

Parthenogenesis—development of an egg without benefit of spermatozoon.

Parthenogenesis, Artificial—initiation of development of an egg by artificial

means.

Parthenogenesis, Natural—maturation of eggs of some forms leads directly

to development without aid of spermatozoa.

Parthenogenetic Cleavage—fragmentation of protoplasm of old and un-

fertilized chick eggs, originally thought to be true cleavage.

Path, Copulation—See Copulation Path.

Path, Penetration—initial direction of sperm entrance into egg, often shift-

ing toward egg nucleus along a new copulation path. (Syn., entrance

path.)

Perforatorium—See A crosome.

Pericardial Cavity—cavity or membrane sac which encloses heart, repre-

senting a cephalic portion of coelom within embryonic body. (Syn.,

parietal cavity.)

Pericardium—thin mesodermal membrane which encloses pericardial cav-

ity and heart.

Perichondrium—mesenchymal layer immediately around forming cartilage.

Perichordal Sheath—thin, mesodermal (sclerotomal), continuous sheet of

tissue immediately around notochord.

Periosteum—mesenchymal layer, often originally perichondrium, which

will be found immediately around forming bone.


Peristomiai Mesoderm—mesoderm of amphibian gastrula derived from

(ventral) lips of blastopore. Opposed to gastral mesoderm.

Peritoneal cavity—body cavity (coelom).

Peritoneum—coelomic mesothelium of abdominal region reinforced by

connective tissue.

Perivitelline Membrane—See Vitelline Membrane.

Perivitelline Space—space between vitelline (fertilization) membrane and

contained egg, generally filled with a fluid.

Pfliiger's Law—dividing nucleus elongates in direction of least resistance.

Phenotype—outward appearance of an organism regardless of its genetic

make-up. Opposed to genotype.

Pigment Layer of Optic Cup—thin outer wall of primary optic cup, poste-

rior to retina, which never fuses with rods and cones of retina.

Pineal—See Epiphysis.

Pituitary—See Hypophysis.

Placode—Plate-like thickening of ectoderm from which arise sensory

or nervous structures (e.g., olfactory placode).

Plane—imaginary two-dimensional surface; may be frontal, sagittal, trans-

verse, median, or lateral.

Plasmosome—a true nucleolus. (See Nucleolus.)

Plectrum—See Columella.

Plexus Choroid—Vascular folds in roof of prosencephalon, diencephalon,

and rhombencephalon.

Poikilothermal—cold-blooded; animals whose body temperatures are sub-

ject to environmental changes because they lack regulating mechanisms.

Opposed to homoiothermal.

Polar—pertaining, in most cases, to animal pole, although may refer to

vegetal pole, or both.

Polar Body—relatively minute, discarded nucleus of maturing oocyte

(generally three). (Syn., polocytes.)

Polarity—axial distribution of component parts; animal and vegetal poles;

stratification.

Pole, Animal—region of egg where polar bodies are eliminated; ectoderm

forming portion of pre-cleaved egg. (Syn., apical or animal hemi-

sphere.)


Pole, Vegetal—region of egg opposite animal pole; region of lowest meta-

bolic rate; pole with greater density of yolk in telolecithal eggs; gen-

erally endoderm-forming region of egg.

Polyembryony—production of several separate individuals from one egg

by an early separation of its blastomeres; possible origin of some

identical twins.

Polyploid—possessing a multiple number of chromosomes, such as triploid

(three times the haploid number), tetraploid (four times the haploid

number), etc. Alwavs more than the normal diploid of the typical

zygote.

Polyspermy—insemination of an egg with more than a single sperm,

occurring generally in chick egg, although but a single sperm nucleus

is functional, in syngamy.

Post-Ana! Gut—posteriorly projecting blind pocket of hindgut, that por-

tion of hindgut posterior to anal plate or proctodeal plate. (Syn., post-

cloacal gut.)

Post-Reduction—maturation in which equational and reductional divisions

occur in that order.

Posterior Tubercle—See Tuberculum posterius.

Potency, Prospective—sum total of developmental possibilities, the full

range of developmental performance of which a given area (or germ)

is capable. Not to be confused with competence.

Preformation—theory that adult is represented in miniature within egg

or sperm and that development is simply enlargement.

Pre-migratory Germ Cell—yolk-laden cells of splanchnopleuric origin

which migrate by way of blood vessels to gonad primordia. Believed

by some to be precursors of gonad stroma or functional germ cells.

Pre-Reduction—maturation in which reductional and equational divisions

occur in that order.

Presumptive—expected or predicted outcome of development of a given

area (e.g., fate of a part in question) based on previous fate mapstudies.

Primary Oocyte—termination of growth phase in maturation of ovum from

oogonial stage, prior to any maturational divisions.

Primary Spermatocyte—stage in spermatogenesis in which division results

in secondary spermatocytes; stage beginning with growth of sperma-

togonia.


Primitive Groove—groove through center of primitive streak, bounded by

primitive folds and terminated anteriorly by primitive pit and pos-

teriorly by primitive plate.

Primordial Germ Cells—diploid cells which are destined to become germcells (e.g., oogonia and spermatogonia). (Syn., primitive germ cells.)

Primordium—See Anlage.

Proctodeum—ectodermal pit in region of future cloaca which invaginates to

fuse with hindgut endoderm to form anal or proctodeal plate, later to

rupture and form anus.

Pronephric Capsule—mesodermal connective tissue covering of pronephric

masses derived from adjacent myotomes and somatic mesoderm.

Pronephric Chamber—portion of amphibian coelomic cavity open ante-

riorly and posteriorly but closed ventrally by development of lungs.

Pronephric Duct—outer portion of pronephric nephrotomes which de-

velops a lumen connected posteriorly with mesonephric or Wolffian

duct. (Syn., segmental duct.)

Pronephric Tubules—lateral outgrowths of the most anterior nephrotomal

masses which acquire cavities in amphibia, connected with pronephric

duct. Possibly become infundibulum of oviduct.

Pronephros—embryonic kidney of all vertebrates, extending from second

to fourth somites of frog embryo and consisting of as many primitive

tubules as somites concerned; completely lost in all adult vertebrates

except a few bony fish. (Syn., head kidney.)

Pronucleus—egg nucleus after polar body formation and sperm nucleus

after entrance of spermatozoon into egg.

Prophase—first stage in mitotic cycle when spireme is broken up into

definite chromosomes, prior to lining up on metaphase (equatorial)

plate.

Prosencephalon—See Forebrain.

Prosocoel—cavity of prosencephalon.

Proximal—nearer the point of reference, toward main body mass.

Pupil—opening into secondary optic vesicle, occluded in part by lens, and

regulated in diameter by ciliary muscles of iris.

Ramus Communicans—connection between sympathetic ganglion and

spinal nerve, as numerous as ganglia in any vertebrate; probably orig-

inating from crest cells. Ramus means branch.


Recapitulation Theory—theory that embryonic development reviews major

steps in evolutionary history. (See qualifications under Biogenetic Law.)

Rectum—narrowed posterior portion of hindgut, lined with thickened en-

dodermal epithelium, which opens directly into cloaca.

Reductional Maturation Division—one of the two important divisions in

the maturation of gametes which results in separation of allelomorphic

(homologous) pairs of chromosomes so that resulting cells are in-

variably haploid. Opposed to equational division. (Syn., meiotic divi-

sion, disjunctional division.)

Regeneration—repair or replacement of lost part or parts, a power gradu-

ally lost in the ontogeny of most animals.

Regions, Presumptive—regions of blastula which, by previous experimen-

tation, have been demonstrated to develop in certain specific direc-

tions under normal ontogenetic influences.

Regulation—reorganization toward the whole; power of pre-gastrula em-

bryos to utilize materials remaining, after partial excision, to bring

about normal conditions; more flexible power than regeneration.

Renal Portal System-—venous system which carries blood to kidneys, in-

volving lateral portions of caval veins (really parts of posterior cardi-

nals), iliacs, and dorso-lumbars. Found in adult amphibia as the most

striking evidences of recapitulation.

Rete Cords—strands of epithelial cells containing many primordial germ

cells which connect with seminiferous tubules and later become vasa

efferentia, in the bird. (Syn., rete testis.)

Retinal Zone—ectodermal derivatives of optic cup consisting of internal

limiting membrane, retinal and lenticular zones, and outer pigmented

layer. Retina proper includes portions from internal limiting membrane

to rods and cones, inclusive.

Rhombencephalon—See Hindbrain.

Saccule—-outer and ventral portion of inner ear from which are derived

cochlea associated with eighth or auditory nerve. (Syn., sacculus.)

Saccus Endolymphaticus—original endolymphatic duct, closed off from

exterior, which (in 20 mm. stage of tadpole) grows up over rhomben-

cephalon to join other sac and form a vascular covering of the brain.

Sachs' Law—all cells tend to divide into equal parts and each new plane

of division tends to intersect the preceding one at right angles.


Sagittal—mesial plane, or any plane parallel to it, dividing right parts

of body from left. Right angles to both frontal and transverse planes.

Sclerotic Coat—tough mesenchymatous and partially cartilaginous coat

outside of choroid coat of vertebrate eye. (Syn., sclera.)

Sclerotome—loose mesenchymal cells proliferated off from inner and

ventral edges of myotomes (5 mm. frog) which contribute to formation

of axial skeleton.

Secondary Oocyte—stage in oogenesis between primary oocyte and ovum;

may be either haploid or diploid, depending upon species considered.

Secondary Spermatocyte—stage in spermatogenesis in which next division

results in haploid spermatids, these spermatocytes being either haploid

or diploid, depending upon species considered. (See Post- and Pre-

reduction.)

Secretory Tubule—portion of kidney tubule actually involved in excretory

process.

Section—generally a slice of an embryo, often of microscopic dimensions,

taken in any one of the various planes such as frontal, transverse, or

sagittal. (See Serial Sections.)

Segmental Plate—See Axial Mesoderm.

Segmentation—repetition of structural pattern; used as synonym for cleav-

age as well as for metamerism.

Segmentation Cavity—cavity of blastula. (Syn., subgerminal cavity, blasto-

coel.)

Semi-Circular Canals—tubular derivatives of utricle lined with ectoderm

from otocyst, which constitute accessory balancing mechanisms of

vertebrates.

Seminal Vesicle—glandular dilation of distal end of ductus deferens

(Wolffian duct) where spermatozoa are temporarily collected prior to

ejaculation.

Semination—act 'of fertilizing by discharge of spermatozoa.

Seminiferous Tubule—tubular divisions of testis derived from rete cords,

covered by a connective tissue theca and containing supporting (Sertoli)

cells and all stages of spermatogenesis.

Sense Plate—narrow band of elevated ectodermal tissue which passes

transversely across anterior end of amphibian embryo, ventral to level

of fused neural folds, with ends of band bending dorsally to merge

with neural folds. Lower margins represent mandibular arch, the


plate giving rise to mucous glands (oral suckers) of amphibia and to

parts of olfactory organs, lens of eye, and possibly to part of inner

ear.

Septum—partition.

Serial Sections—thin (often of microscopic dimensions) sections of em-

bryos which are mounted on slides in order of their removal from the

embryo, so that a study in sequence will provide an understanding

of all organ systems from one region of embryo to the other.

Sertoli Cell—derivative of sexual cords of testis, found within seminiferous

tubule and functionally similar to follicle cell in ovary in that it is the

nutritive, supporting, or nurse cell of the maturing spermatozoa. The

heads of adult spermatozoa may be seen embedded in the cytoplasm of

Sertoli cells.

Sex Cell Cord—division of sex cell ridge or gonad primordium, not to be

confused with sexual (rete) cords.

Sex Determination—See Determination of Sex.

Sexual Cords—derivatives of germinal epithelium from which they become

separated and give rise to bulk of gonads of both sexes.

Sexual Cords of the Ovary—sex cords of the originally indifferent gonad

primordium which form only cords of ovary, the functional follicles

coming from germinal epithelium.

Sexual Cords of the Testis—sex cords of the originally indifferent gonad

primordium which give rise to seminiferous tubules of testis, forming

a rather solid mesenchymatous reticulum when cavities begin to appear

lined with spermatogonia (from primordial germ cells) and Sertoli cells,

the whole constituting seminiferous tubules.

Sheath, Myelin—myelin covering of axons in so-called white matter of

spinal cord.

Sinus Venosus—point of fusion of vitelline veins of amphibian embryo

bilaterally symmetrical and related to ducts of Cuvieri and ductus

venosus.

Skeletogenous Sheath—sclerotomal cells which first form a continuous layer

around both notochord and nerve cord.

Skin—See Dermis and Epidermis. (Syn., integument.)

Somatic—relating to body in contrast to germinal cells; or relating to

outer body in contrast to inner splanchnic mesoderm.

Somatoblast—blastomeres with specific germ layer predisposition, i.e., ecto-

dermal somatoblasts.


Somatopleure—layer of somatic mesoderm and closely associated ectoderm,

extension of which (from body wall) gives rise to both amnion and

chorion.

Somite—blocks of paraxial mesoblast, metamerically separated by trans-

verse clefts, derived from enterocoelic or gastral mesoderm and

giving rise to dermatome, myotome, and sclerotome.

Spawning—act of expelling eggs from uteri of anamniota (e.g., amphibia).

Sperm—germ cell characteristically produced by the male. (Syn., sperma-

tozoon, sperm cell, male gamete, spermatosome.)

Spermatid—products of the second maturation division in spermatogenesis,

the spermatids having certain cytological characteristics and being in-

variably haploid; cells which go through a metamorphosis into func-

tionally mature spermatozoa.

Spermatocyte—stages in spermatogenesis between the time the primordial

germ cell (spermatogonium) begins to grow, without division, until

after the division which results in spermatids. (See Primary Spermato-

cyte, Secondary Spermatocyte.)

Spermatogenesis—entire process which results in maturation of sperma-

tozoon.

Spermatogonium—primordial germ cell of male gonad, indistinguishable

from somatic cells, both of which are diploid; stage prior to maturation

when the presumptive spermatozoon undergoes rapid multiplication by

mitosis.

Spermatophore—sperm-bearing bundle, such as that which is shed by male

urodele, the bundles later to be picked up by cloacal lips of female.

Spermatosphere—See Idiosome.

Spermatozoon—functionally mature male gamete. (Syn., sperm.)

Spina Bifida—split tail, generally involving spine, in developing embryo

caused by a variety of environmental conditions, most of which act

through interference with normal gastrulation and neurulation.

Spinal Cord—that portion of central nervous system, excluding brain,

which is derived from epithelial and neural ectoderm of original

blastula, consisting of ependyma, glia, neuroblasts and their derivatives,

and connecting cells.

Spindle—group of fibers between centrosomes during mitosis, to which

chromosomes are attached and by means of which (mantle fiber por-

tion) chromosomes are drawn to their respective poles.


Spinous Process—prolongation of neural processes fused dorsally to neural

canal; becomes dorsal spine of vertebra.

Spiracle—short funnel between body wall and operculum on left side of

head of frog tadpole, the only exit for water passing through gill

chambers to exterior.

Spireme—continuous chromatin thread characteristic of so-called resting

cell nucleus. Existence questioned by current cytologists.

Splanchnic—refers to viscera, opposed to somatic or body.

Splanchnic Mesoderm—visceral mesoderm, or that nearest embryonic axis

in lateral plate.

Splanchnocoel—that portion of enterocoel (of Amphioxus) which lies

between somatic and splanchnic mesoderm within body. (Syn., coelom.)

Splanchnocranium—that portion of skull which is preformed in cartilage

and which arises from the first three pairs of visceral arches. Opposed

to neurocranium.

Splanchnopleure—layer of endoderm and inner (splanchnic) mesoderm

within which develop the numerous blood vessels of area vasculosa and

later yolk sac septa; layers within the body which give rise to lining

and to musculature of alimentary canal.

Spongioblasts-—cells of mantle layer of developing spinal cord destined

to form merely supporting tissue.

Stereoblastula—solid blastula as found in Crepidula.

Stomodeum—ectodermal invagination (pit) which fuses with pharyngeal

endoderm to form oral plate, which later ruptures to form margins of

mouth cavity. Stomodeal portion of mouth lining is therefore ecto-

dermal.

Stroma—mesodermally derived, medullary, supporting tissues of an organ.

Sub-Germinal Cavity—See Blastocoel, Segmentation Cavity.

Sub-Notochordal Rod or Bar—hypochordal rod of amphibian embryo,

found dorsal to midgut. Transitory.

Sucker—adhesive, connecting organ of oral region (larval stage).

Sustentacular Cell—cell which provides nourishment for another, such as

Sertoli or follicle cells of gonads.

Sylvius, Aqueduct of—See Aqueduct of Sylvius.

Sympathetic System—originating either from mesenchymal element arising

in situ or, more probably, from ectodermal elements emanating from


neural crests, to organize as a chain of ganglia near dorsal aorta and

controlling involuntary (visceral) musculature.

Synapsis—union, such as the lateral (parasynapsis) or terminal (telosynap-

sis) union of embryos; or pairing of homologous chromosomes.

Synaptene Stage—stage in maturation between leptotene and synizesis (con-

traction) stage wherein chromatin is in form of long threads, inter-

twined in homologous pairs. (Syn., zygotene, amphitene.)

Syncytium—nuclei and cytoplasm without cellular boundaries; multinu-

cleate protoplasm without cell boundaries.

Syngamy—specifically the fusion of the gamete pronuclei, but also the

union of gametes at fertilization. (Syn., zygotogenesis, fertilization.)

Synizesis—stage in maturation between synaptene and pachytene when

chromatin threads are short and thick and ends away from centrosome

are tangled.

Telencephalon—portion of forebrain (ventricle) anterior to a plane which

includes posterior side of choroid plexus and anterior side of optic

recess of 5 mm. frog embryo. Gives rise to torus transversus (anterior

commissure), cerebral hemispheres, corpora striata, anterior choroid

plexus, olfactory lobes, lateral ventricles, and part of foramina of

Monro.

Telobiosis—fusion of embryos end-to-end. (Syn., parabiosis.)

Telocoel—cavity of telencephalon.

Telolecithal—See Egg, telolecithal.

Telophase—last phase in mitosis when respective chromosome groups

have reached their respective astral centers and are beginning to re-

form a resting cell nucleus; stage often accompanied by beginning of

cytoplasmic division.

Telosynapsis—end-to-end fusion of chromosomes. (Syn., parasynapsis.)

Teratology—study of causes of monster formation.

Tetrads—paired (homologous) chromosomes which have become dupli-

cated longitudinally in anticipation of the meiotic (reductional) divi-

sion. When viewed from end will appear as a group of four chromo-

somes, hence a tetrad.

Thalamus—dorso-lateral wall of diencephalon which becomes thickened

by development of fibers passing from cord to more posterior parts

of cerebral hemispheres.


Theca externa-—outermost of coverings of ovarian follicle, rather loose con-

nective tissue with abundant blood supply. Continuous with perito-

neum.

Theca interna—layer of connective tissue consisting of closely packed

fibers, possibly some of smooth muscle, immediately external to egg.

Consists of cyst wall.

Thymus—derivatives of first pair of branchial pouches of frog embryo

which separate from pouches (12 mm.) and migrate to a position

posterior to auditory capsules near surface of the head. Endocrine

functions.

Thyroid (Body or Gland)—originates as an endodermal thickening in floor

of pharynx between second pair of visceral arches; evaginates to form a

vesicle temporarily connected with gut by a duct; separates from gut;

becomes divided; and migrates to position near hyoglossus muscle.

Somewhat similar history in all vertebrate embryos. Endocrine func-

tion.

Tissue Culture—in vitro culturing of isolated tissues; excision of tissues or

organs and their maintenance in an artificial medium, generally consist-

ing in part of embryonic extracts or blood plasma.

Tongue—solid mesodermal mass, covered with endoderm, derived by cell

proliferation from floor of pharynx beginning in the 9 mm. frog tadpole.

Tonsils—lymphatic structures derived from endoderm and mesoderm of

second pair of visceral pouches.

Torus Transversus—thickening in median ventro-anterior wall of lamina

terminalis of telencephalon, just exterior to optic recess, representing

rudiment of anterior commissure.

Totipotency—related to theory that isolated blastomere is capable of

producing a complete embryo.

Trachea—that portion of respiratory tract between larynx and lung buds,

lined with endoderm, probably derived from posterior portion of origi-

nal laryngotracheal groove.

Tracheal Groove—Syn., laryngotracheal groove.

Transplant—an embryonic area (cell, tissue, or organ) removed to a dif-

ferent environment.

Transverse—a plane (or sections) which divides antero-posterior axis at

right angles, separating more anterior from more posterior. (Syn.,

cross section, but this synonym is not generally satisfactory.)


Transverse Neural Fold—continuation of lateral neural folds (ridge) of early

frog embryo around anterior neuropore. (Syn., transverse medullary

fold or ridge.)

Trigeminal Ganglion—cranial (V) ganglia which consist of motor and

sensory portions and arise from segments of the most anterior crest

in conjunction with cells from inner (ganglionic) portion of corre-

sponding placode. Give rise to ophthalmic, mandibular, and maxillary

branches, associated with rhombencephalon at level of greatest width

of fourth ventricle.

Trochlearis Nerves—cranial (IV) motor nerves which arise from dorsal sur-

face of brain near isthmus, coming from medullary neuroblasts and

innervating superior oblique muscles of eye.

Truncus Arteriosus—anterior continuation of buibus arteriosus beneath

foregut, divided in antero-posterior direction by a septum which is

continuous through buibus to ventricle; gives off external carotids to

mandibular arches and second, third, and fourth aortic arches which

join dorsal aorta. (Syn., ventral aorta.)

Tuberculum Posterius—a thickening in floor of brain at region of anterior

end of notochord, representing posterior margin of diencephalon.

Tubo-tympanic Cavity—remnants of dorsal parts of first pair of visceral

(hyomandibular) pouches and lateral walls of pharynx, connecting

pharynx and middle ear, represented by Eustachian tube of adult bird

or mammal.

Tubules—See under specific names such as Collecting, Mesonephric, Pro-

nephric. Seminiferous.

Tunica Albuginea—See Albuginea of Testis.

Tympanic Cavity—cavity of middle ear, a vestige of hyomandibular pouch.

(See Tubo-tympanic Cavity.)

Tympanic Membrane—membrane made up of ectoderm, mesenchyme, and

endoderm which separates tympanic cavity from exterior. (Syn., ear

drum.)

Urinary Bladder-—endodermally lined vesicle derived from hindgut, ho-

mologous to allantois of chick. Connected with mesonephric (excre-

tory) ducts of frog only through cloaca.

Uriniferous Tubule—functional kidney tubule of mesonephros.

Urodele—tailed amphibia (e.g., salamanders). (Syn., caudata.)


Urogenital Duct—ducts which open into cloaca of male amphibia and

convey both excretory and genital products, derived from mesoneph-

ric (Wolffian) ducts.

Urogenital System—entire excretory and reproductive systems, some em-

bryonic parts of which degenerate before hatching. Shows various

degrees of common origin and ultimate function. (See specific excretory

and reproductive components.)

Urostyle—fused skeletogenous elements of the last two somites in frog

embryo which surround end of notochord as cartilage and finally ossify.

Utricle—a vesicle, generally referring to superior portion of otocyst which

gives rise to the various semi-circular canals of the ear, and into which

these canals open. Lined with ectoderm.

Vasa Deferentia—mesonephric or Wolffian ducts of frog, which persist

as male gonoducts of bird and mammal, connecting with testes through

vasa efferentia and epididymis and functioning as sperm ducts after

degeneration of embryonic mesonephros and development of gonads.

(Sing., vas deferens.)

Vasa Efferentia—ducts which convey frog sperm from collecting tubules

through mesorchium to Malpighian corpuscles of mesonephric kidney;

derived from rete cords and connected with mesonephric tubules of

anterior (sexual) half of the mesonephric or Wolffian body.

Vegetal Pole—pole of a telolecithal egg where there is greatest concen-

tration of yolk, usually opposite animal pole and location of ger-

minal vesicle. (Syn., vegetal or vegetative hemisphere; abapical or

antipolar hemisphere.) (See Animal Pole.)

Vein—See under specific names.

Velar Plate—folds or flaps developing anterior and posterior to branchial

regions of frog (anuran) embryo derived from pharyngeal wall and

serving as a gross sifting organ between pharynx and gill (branchial)

chamber.

Velum Transversum—depressed roof of telencephalon just anterior to

lamina terminalis, which later becomes much folded and vascular as

anterior roof of third ventricle.

Vena Cava Anterior—junction of inferior jugular (anterior cardinal) and

subclavian and vertebral veins which empty into ductus Cuvieri, and

later the right auricle. (Syn., superior vena cava, superior caval veins.)


Vena Cava Posterior—single median ventral vein which represents rem-

nant of anterior right cardinal and which later receives hepatic vein

prior to joining ductus Cuvieri, and later joins right auricle directly.

Ventral—belly surface. Ventrad means toward belly surface.

Ventral Mesentery—double layer of mesoblast which connects alimentary

canal with splanchnopleure in embryo.

Ventricle III—main cavity (diocoel) of forebrain, related to paired lateral

ventricles or telocoels, by way of foramina of Monro.

Ventricle IV—main cavity of hindbrain (rhombencephalon) connected

anteriorly with aqueduct of Sylvius and posteriorly with neural canal,

having as a roof the vascular posterior choroid plexus.

Ventricle, Lateral—See Lateral Ventricles of the Brain.

Ventricle of the Heart—chamber of the heart, single in frog and very mus-

cular, developing from anterior myocardium and provided with valves;

connected with bulbus arteriosus anteriorly.

Vertebra—derivatives of sclerotome which surround nerve cord and noto-

chord, and finally incorporate notochord by chondrification and

ossification (centrum).

Vertebral Arch—See Neural Arch.

Vertebral Plate—See Axial Mesoderm. (Syn., segmental plate.)

Vesicle, Germinal—nucleus of egg while it is a distinct entity and before

elimination of either of the polar bodies.

Visceral—pertaining to viscera.

Visceral Arches—mesodermal masses (usually six pairs) between visceral

pouches and lateral to pharynx of all vertebrate embryos, including

mandibular, hyoid, and four branchial arches. Each arch is bounded

by endoderm on pharyngeal side and ectoderm on outside. (Syn,,

visceral arches III to VI are also called branchial arches I to IV,

respectively; pharyngeal arch.)

Visceral Clefts—slit-like openings between pharynx and outside, found in

vertebrate embryos on either side of visceral arches II to V, or less,

consisting of peripheral lining of ectoderm and mesial lining of

endoderm. (Syn., pharyngeal, and some may be called gill or branchial

clefts.)

Visceral Furrow—ectodermal invaginations which may meet endodermal

pharyngeal evaginations to form visceral clefts. (Syn., visceral groove.)

Visceral Groove—See Visceral Furrow.


Visceral Mesoderm—See Splanchnic Mesoderm, Splanchnopleure.

Visceral Plexus—aggregation of sympathetic neurons which control viscera,

having migrated posteriorly from tenth (vagus) cranial ganglia.

Visceral Pouch—endodermal evagination of pharynx which, if it meets cor-

responding visceral furrow, often breaks through to form visceral cleft.

(Syn., pharyngeal pouch.)

Vital Stain—localized staining of living embryonic areas with vital, non-

toxic dyes.

Vitalism—a philosophical approach to biological phenomena which bases

its proof on present inability of scientists to explain all phenomena of

development. Idea that biological activities are directed by forces

neither physical nor chemical but which must be supra-scientific or

supernatural. Effective guidance in development by some non-material

agency. (See Mechanism.)

Vitelline—pertains to yolk (e.g., vitelline vein brings blood from yolk;

vitelline membrane is that which covers yolked egg).

Vitelline Artery—paired off shoots of dorsal aorta which take blood to

belly yolk of early embryo, later to become coeliac and mesenteric

arteries.

Vitelline Membrane—delicate, outer, non-living egg covering derived while

egg is still within ovary, probably by joint action of egg and its

follicle cells; probably same membrane that is elevated as the fertiliza-

tion membrane after successful insemination. (Syn., fertilization mem-brane.)

Vitelline Substance—yolk.

Vitelline Vein—paired veins, first to be formed in embryo, found in ventro-

lateral splanchnopleure, carrying nutritious blood from yolk region

to their junction with sinus venosus prior to the full development and

function of heart.

Vitreous Humor—the rather viscous fluid of eye chamber posterior to lens,

formed by cells budded from retinal wall and from inner side of lens,

hence ectodermal and probably also mesenchymal in origin. (See

Aqueous Humor.)

Wolffian Body—See Mesonephros.

Wolffian Duct—See Mesonephric Duct, Urogenital Duct, Vasa Deferen-

tia.


Yolk—highly nutritious food (metaplasm) consisting of non-nucleated

spheres and globules of fatty material found in all except alecithal eggs.

Yolk Nuclei—darkly staining chromatin-like substances within cytoplasm

of young (immature) eggs around which yolk is accumulated during

growth phase of oogenesis. May be derived from nucleoli which escape

from nucleus.

Yolk Plug—a plug formed by large yolk cells which are too large to be

incorporated immediately in floor of archenteron of amphibian em-

bryo, hence are found protruding slightly from blastopore. Size of plug

is often used to determine approximate stage of gastrulation.

Zone, Marginal—presumptive chorda-mesodermal complex at junction of

roof and floor of early gastrula. (Syn., germ ring.)

BiLliodrapliy for Frog Emtryology

Contributions to this book on Frog Embryology have been made from

many of the following references. It is therefore quite a complete bibli-

ography on normal frog development. Those interested in more detailed

references to specific subjects should consult the author's "Experimental

Embryology."

Books

Allen, B. M., 1930, "The Early Development of Organ Anlagen in

Amphibia," California, Stanford University Press, Contributions to

Marine Biology, 204-212.

Anglas, J., 1904, "Observations sur les metamorphoses internes des

batraciens anoures," Grenoble, Assoc. Frang. p. 1. Avanc. d. Sciences,

33 Sess., p. 855.

Arey, L. B., 1946, "Developmental Anatomy," 5th ed., Philadelphia, W. B.

Saunders Co.

Barth, L. G., 1949, "Embryology," New York, The Dryden Press, Inc.

Brachet, A., 1935, "Traite d'embryologie des Vertebres," Paris, Masson et

Cie.

Brachet, Jean, 1947, "Embryologie Chimique," Paris, Masson et Cie.

Butschli, O., 1915, "Bemerkung zur mechanischen Erklarung der Gastrula-

Invagination," Sitz. ber. Heidelberg Akad. Wiss., vol. 6, Part II, pp. 1-13.

Celestino da Costa, A., 1938, "Elements d'Embryologie," Paris, Masson et

Cie.

Dalcq, A., 1938, "Form and Causality in Early Development," London,

Cambridge University Press.

, 1935, "L'Organisation de I'oeuf chez les chordes. Etude d'embry-

ologie causale," Paris, Gauthier-Villars.

DeBeer, G. R., 1926, "The Comparative Anatomy, Histology, and Develop-

ment of the Pituitary Body," London, Oliver & Boyd.

Dickerson, M. C, 1906, "The Frog Book," New York, Doubleday &

Company, Inc.

Ecker, A., 1889, "Anatomy of the Frog," Oxford, Clarendon Press.

Furbinger, M., 1877, "Zur Entwicklungs der Amphibienniere," Heidel-

berg.

311

312 BIBLIOGRAPHY FOR FROG EMBRYOLOGY

Gaupp, E., 1904, "Anatomic des Frosches," Braunschweig, Vieweg &Sohn.

Goette, A., 1875, "Die Entwicklungsgeschichte der Unke {Bombinator

igneus) ," Leipzig, Leopold Voss.

Goodrich, E. S., 1930, "Studies on the Structure and Development of

Vertebrates," New York, The Macmillan Co.

Held, H., 1909, "Entwicklung des Nervengewebe bei den Wirbeltiere,"

Leipzig.

Hertwig, O., 1909, "Handbuch der vergleichenden und experimentellen

Entwicklungslehre der Wirbeltiere," 6 vols., Jena, Fischer.

Hinsberg, B., 1901, "Die Entwicklung der Nasenhohle und experimentellen

Entwicklungslehre der Wirbeltiere," Jena.

His, Wilhelm, 1874, "Unsere Korperform und des physiologische Problem

ihrer Entstehung," Leipzig, Vogel.

Holmes, S. J., 1927, "Biology of the Frog," 4th ed., rev.. New York,

The Macmillan Co.

Huettner, A. F., 1949, "Fundamentals of Comparative Embryology of the

Vertebrates," New York, The Macmillan Co.

Jordan, H. E., and J. E. Kindred, 1948, "A Textbook of Embryology,"

New York, D, Appleton-Century Co.

Kellicott, W. E., 1913, "Outlines of Chordate Development," New York,

Henry Holt & Company, Inc.

, 1913, "A Textbook of General Embryology," New York, Henry

Holt & Company, Inc.

Kingsley, J. S., 1907, "The Frog, An Anurous Amphibian," New York,

Henry Holt & Company, Inc.

Marshall, A. M., 1893, "Vertebrate Embryology," London, Putnam & Co.,

Ltd.

, 1885, "The Frog: An Introduction to Anatomy, Histology, and

Embryology," New York, The Macmillan Co.

Marshall, F. H. A., 1922, "Physiology of Reproduction," 2d ed., rev., NewYork, Longmans, Green & Company.

McEwen, R. F., 1949, "Vertebrate Embryology," New York, Henry Holt &Company, Inc.

Minot, C. S., 1910, "A Laboratory Text-book of Embryology," Phila-

delphia, The Blakiston Company.Morgan, T. H., 1897, "Development of the Frog's Egg: Introduction to

Experimental Embryology," New York, The Macmillan Co.

Needham, J., 1942, "Biochemistry and Morphogenesis," 6th ed., London,Cambridge University Press.

Noble, G. K., 1931, "The Biology of the Amphibia," New York, TheMcGraw-Hill Book Company, Inc.

ARTICLES 313

Pasteels, Jean, 1939, "Sur I'origine du materiel caudal des Urodeles,"

Bruxelles, Palais des Academies.

Pope, C. H., 1944, "Amphibians and Reptiles of the Chicago Area,"

Chicago, Chicago Natural History Museum.

Reese, A. M., 1947, "The Lamina Terminalis and the Preoptic Recess in

Amphibia," Washington, D. C, Smithsonian Miscellaneous Collection,

vol. 106.

Richards, Aute, 1931, "Outline of Comparative Embryology," New York,

John Wiley & Sons, Inc.

Rugh, Roberts, 1949, "A Laboratory Manual of Vertebrate Embryology,"

Minneapolis, Burgess Publishing Company.

, 1948, ""Experimental Embryology; A Manual of Techniques and

Procedures," Minneapolis, Burgess Publishing Company.

Shumway, W., 1935, "Introduction to Vertebrate Embryology," 3d ed.,

rev.. New York, John Wiley & Sons, Inc.

Thompson, D'Arcy W., 1942, "On Growth and Form," London, Cambridge

University Press.

Waddington, C. H., 1940, "Organizers and Genes," New York, The Mac-millan Co.

Waterman, A. J., 1948, "A Laboratory Manual of Comparative Verte-

brate Embryology," New York, Henry Holt & Company, Inc.

Weiss, Paul, 1939, "Principles of Development; A Text in Experimental

Embryology," New York, Henry Holt & Company, Inc.

Wieman, H. L., 1949, "An Introduction to Vertebrate Embryology," NewYork, The McGraw-Hill Book Company, Inc.

Wright, A. A., and A. H. Wright, 1949, "Handbook of Frogs and Toads

of the United States and Canada," 3d ed., Ithaca, N. Y., Comstock

Publishing Co., 1949.

Articles

Adelmann, H. B., 1936, The problem of cyclopia, Quart. Rev. Biol., Part

I, 11:161; Part II, 11:284.

Alderman, A. L., 1935, The determination of the eye in the anuran,

Hyla regilla, J. Exper. Zool., 70:205.

Allen, B. M., 1919, The development of the thyreoid glands of Bufo and

their normal relation to metamorphosis, /. Morphol., 32:489.

, 1907, An important period in the 'history of the sex-cells of Ranapipiens, Anat. Anz., 31:339.

Assheton, R., 1905, On growth centres in vertebrate embryos, Anat. Anz.,

27:125; 156.


Atlas, Meyer, 1938, The rate of oxygen consumption of frogs during

embryonic development and growth, Physiol. ZooL, 11:278.

, 1935, The effect of temperature on the development of Ranapipiens, Physiol. ZooL, 8:290.

Atwell, W. J., 1919, The development of the hypophysis of the Anura,

Anat. Rec, 15:73.

Bataillon, E., 1910, L'embryogenese, complete provoquees chez les Am-phibiens par piqure de Toeuf vierge, larves parthenogenesiques de Rana

jusca, Comp. rend. Acad, de sc, 150:996.

, 1891, Recherches anatomiques et experimentales sur la meta-

morphose des amphibiens anoures, Ann. d. I. Univers. de Lyon, 2:1.

Beneden, V., and V. Bambeke, 1927, Extrait des Archives de Biologic,

/. Anat., p. 486

Bialaszewicz, K., 1909, Beitrage zur Kenntnis der Wachstumsvorgange bei

Amphibienembryonen, Bull. Acad. Sci. Cracovie (review in Arch. j.

Entwcklngsmechn. d. Organ., 28:160).

Born, G., 1884, Ueber den Einfluss der Schwere auf des Froschei, Bresl.

drztl. Ztschr., 15:185.

Bouin, M., 1901, Histogenese de la glande genitale femelle chez Ranatemporaria. Arch, de biol., Paris, 17:201.

Boyd, E. M., 1938, Lipoid substances of the ovary during ova production

in Rana pipiens, J. Physiol., 91:394.

Brachet, A., 1911, Etudes sur les localisations germinales et leur poten-

tialite reelle dans I'oeuf parthenogenetique de Rana jusca, Arch, de

biol, Paris, 26:337.

, 1905, Gastrulation et formation de I'embryon chez les Chordes,

Anat. Anz., 27:212; 239.

Braem, F. von, 1898, Epiphysis und Hypophysis von Rana, Ztschr. wiss.

ZooL, 63:433.

Broman, I., 1907, Uber Bau und Entwicklung der Spermien von Rana

fusca. Arch. mikr. Anat., 70:330.

Brown, M. E., 1946, The histology of the tadpole tail during metamorphosis,

with special reference to the nervous system, Am. J. Anat., 78:79.

Burns, R. K., Jr., 1934, The effect of the removal of the pronephros and

duct upon development, Anat. Rec, 58: suppl. 7.

Cameron, J. A., 1941, Primitive blood-cell generations in Amblystoma,

/. MorphoL, 68:231.

Carnoy, J. B., and H. Lebrun, 1900, La vesicule germinative et les globules

polaires chez les batraciens, Cellule, 17:199.

Cooper, Ruth Snyder, 1943, An experimental study of the development of

ARTICLES 3 1 5

the larval olfactory organ of Rami pipiens Schreber, /. Exper. ZooL,

93:415.

Corning, H. K. von, 1899, Uber einige Entwicklungsvorgange am Kopf

der Anuren, Morphol. Jahrh., 27:173.

Dalcq, A. M., 1940, Contributions a i'etude du Potcntiel Morphogenetique

chez les Anoures: 1. Experiences sur la zone marginale dorsale et le

plancher du blastocoele, Arch, de bioL, Paris, 51:387.

, 1933, La determination de la vesicule auditive chez le discoglosse,

Arch, d'anat. micr., 29:389.

D'Angelo, S. A., 1941, An analysis of the morphology of the pituitary and

thyroid glands in amphibian metamorphosis. Am. J. Anat., 69:407.

, and H. A. Charipper, 1939, The morphology of the thyroid gland

in the metamorphosing Rana pipiens, J. Morphol., 64:355.

Delpino, Itala, 1932, Ricerche sperimentali sullo sviluppo degli organi

adesivi in Rana escidenta. Arch. zool. ital., 17:401.

Desclin, Leon, 1927, Etude de la localisation des ebauches ganglionnaires

craniennes dans le germe de Rana fusca. Arch, de bioL, Paris, 37:485.

Durham, H. E., 1886, Note on the presence of the neurenteric canal in

Rana, Quart. J. Micr. Sci., 26:509.

Du Shane, G. P., 1938, Neural fold derivatives in the Amphibia: Pigment

cells, spinal ganglia and Rohon-Beard cells, /. Exper. Zool., 78.

Dustin, A. P., 1908, Recherches sur I'origine des gonocytes chez les

Amphibiens, Arch, de bioL, Paris, 23:411.

Elliott, A. L M., 1907, Some facts in the later development of the frog,

Rana temporaria: L The segments of the occipital region of the skull.

Quart. J. Micr. Sci., 51:647.

Erlanger, R. von, 1890, Ueber den Blastoporus der anuran Amphibien,

sein Schicksal und seine Beziehungen zum bleidenden After, Zool.

Jahrb., 4:239.

Etkin, W., 1936, The phenomena of anuran metamorphosis: IIL The

development of the thyroid gland, /. Morphol., 59:69.

Farrington, O. C, 1892, The nephrostomes of Rana, Tr. Conn. Acad.,

8:309.

Federow, V., 1910, Uber die Entwickelung der Lungenvene, Anat. Heft.,

40:529.

Field, H. H., 1891, Contributions from the Zoological Laboratory. XXVILThe development of the pronephros and segmental duct in Amphibia,

Bull. Mus. Comp. Zool. Harvard, 21:201.

Filatow, D. P., 1904, Zur Entwickelungsgeschichte des Exkretionssystems

der Amphibien, Anat. Anz., 25:33.


Foxon, G. E. H., 1947, Circulation in the frog, Nature, 159:236.

Giannelli, L., 1903, Sulle prime fasi di sviluppo del pancreas negli Anfibii

anuri (Rana esculenta) , Monit. zool. ital., 14:33.

Goppert, E., 1891, "Die Entwicklung und das spatere Verhalten des

Pankreas der Amphibien, Morphol. Jahrb., 17:100.

Greil, A., 1905, Uber die Anlage der Lungen, sowie der ultimobranchialen

(postbranchialen, supraperikardialen) Korper bei anuren Amphibien,

Anat. Heft., 29:445.

Hall, R. W., 1904, Contributions from the Zoological Laboratory, No. 150,

The development of the mesonephros and the MuUerian ducts in Am-phibia, Bull. Mus. Comp. Zool. Harvard, 45, No. 2.

Harrison, R. G., 1904, Experimentelle Untersuchungen liber die Entwick-

lung der Sinnesorgane der Seitenlinie bei den Amphibien, Arch. mikr.

Anat., 63:35.

Hegre, E. S., 1946, The developmental relationship between the nervous

and epithelial components of the hypophysis, /. Exper. Zool., 101:65.

Helff, O. M., 1930, Studies in amphibian metamorphosis: VIII. The role of

the urostyle in the atrophy of the tail, Anat. Rec, Al'All

.

Hempstead, M., 1901, Development of the lungs in the frogs Ranacatesbiana, R. sylvatica, and R. virescens, Science, 12.

Holtfreter, J., 1943, 1944, A study of the mechanics of gastrulation, /.

Exper. Zool., Part I, 94:261; Part II, 95:171.

, 1944, Experimental studies on the development of the Pronephros,

Rev. canadienne de biol., 3:220.

Hoskins, E. R., and M. M. Hoskins, 1920, The inter-relation of the thy-

reoid and hypophysis in the growth and development of frog larvae,

Endocrinology , 4: 1

.

Howland, R. B., 1926, Regeneration of the segmental duct and experimental

acceleration of growth of the mesonephros in Amblystoma punctatum,

J. Exper. Zool, 44:327.

Hoyer, H., 1905, Ueber das Lymphgefassystem der Froschlarven, Verhandl.

d. anat. Gesellsch., 19:50.

Janes, R. G., 1938, Studies on the amphibian digestive system: III. Theorigin and development of pancreatic islands in certain species of Anura,

/. Morphol, 62:375.

Jenkinson, J. W., 1909, On the relation between the symmetry of the egg,

the symmetry of segmentation, and the symmetry of the embryo in the

frog, Biometrika, 7:148.

Kindahl, M., 1938, Zur Entwicklung der Exkretionsorgane von Dipnoern

und Amphibien, Acta, zool org., 19:H-1.

King, H. D., 1905, Experimental studies on the eye of the frog embryo.

Arch. f. Entwcklngsmechn. d. Organ., 19:85.

ARTICLES 317

Knouff, R. A., 1935, The developmental pattern of ectodermal placodes in

Rami pipiens, J. Comp. Neurol., 62:17.

, 1927, The origin of the cranial ganglia of Rana, J. Comp. Neurol.,

44:259.

Knower, H. McE., 1908, The origin and development of the anterior

lymph hearts and the subcutaneous sacs in the frog, Anat. Rec, 2:59.

Kopsch, Fr., 1895, Beitrage zur Gastrulation beim Axolotl- und Froschei,

Verhandl. d. anat. Gesellsch., 9:181.

Lams, H., 1907, Contribution a I'etude de la genese du vitellus dans

Tovule des Amphibiens {Rana temporaria) , Arch, d'anat. niicr., 9:607.

Lebrun, H., 1902, La vesicule germinative et les globules polaires chez

les Anoures, Cellule, 19:315.

Lewis, W. H., 1904, Experimental studies on the development of the eye

in amphibia: L On the origin of the lens, Am. J. Anat., 3:505.

Lindeman, V. F., 1929, Integumentary pigmentation in the frog, Rana

pipiens, during metamorphosis, with especial reference to tail-skin

histolysis, Physiol. Zool., 2:225.

Marcelin, R. H., 1903, Histogenese de I'Epithelium intestinal chez la

Grenouille (Rana esculenta) , Rev. Suisse zool., 11:269.

Marx, A., 1925, Experimentelle Untersuchungen zur Frage der Deter-

mination der Medullarplatte, Arch. mikr. Anat., 55.

Maurer, F. von, 1890, Die erste Anlage der Milz und das erste Auftreten

von lymphatischen Zellen bei Amphibien, Morphol. Jahrb., 16:203.

, 1888, Die Kiemen und ihre Gefasse bei anuren und urodelen

Amphibien, und die Umbildungen der beiden ersten Arterienbogen bei

Teleostiern, Morphol. Jahrb., 14:175.

, 1887, Schilddriise, Thymus und Kiemenreste der Amphibien,

Morphol. Jahrb., 13:296.

Maximow, A., 1910, Ueber embryonale Entwickelung der Blutzellen bei

Selachiem und Amphibien, Verhandl. d. anat. Gesellsch., 24:64.

Merwin, R., 1945, Some group effects on the rate of cleavage and early

development of frog eggs (Rana pipiens), Physiol. Zool., 18:16.

, and W. C. AUee, 1943, The effect of low concentrations of carbon

dioxide on the cleavage rate in frog's eggs, Ecology, 24:61.

Miller, R. A., and S. R. Detwiler, 1936, Comparative studies upon the

origin and development of the brachial plexus, Anat. Rec, 65:273.

Morgan, T. H., 1906, The origin of the organ-forming materials in the

frog's embryo, Biol. Bull, 11:124.

Nicholas, J. S., 1945, Blastulation, its role in pregastrula organization in

Amblystoma punctatum, J . Exper. Zool., 100:265.

Nishibe, M., 1928, On the cultivation of kidney tissue from the adult toad.

Arch. f. exper. Zelljorsch., 7:87-97.


Nussbaum, M. von, 1895, Zur Mechanik du Eiablage bei Ratia jusca, Arch.

mikr. Anat., 46:479.

O'Conner, R. J., 1940, The evolutionary significance of the embryology

of the amphibian nephric system, /. Anat., 75: 95.

Pasteels, Jean, 1943, Fermeture du blastopore, anus et intestin caudal chez

les Amphibiens Anoures, Acta Neerland. morphol., 5:11.

, 1943, Proliferations et croissance dans la gastrulation de la queue

des Vertebres, Arch, de bioL, Paris, 54:2.

-, 1942, New observations concerning the maps of presumptive

areas of young amphibian gastrula (Amblystoma and Discoglossus)

/. Exper. ZooL, 89:255.

Pezard, A., and R. M. May, 1937, Les terminaisons nerveuses du muscle

conturier de la Grenouille et la question de sa partie aneurale, Ann. de

physiol., 13:460.

Pollister, A. W., and J. A. Moore, 1937, Tables for the normal development

of Rana sylvatica, Anat. Rec, 68:489.

Porter, K. R., 1939, Androgenetic development of the egg of Rana pipiens,

Biol. Bull., 77:233.

Reed, Margaret, 1905, The formation of the interior cells of the segmen-

tation of the frog's egg, Biol. Bull., 8:189.

Roux, W. von, 1893, Ueber die ersten Teilungen des Froscheies und ihre

Beziehungen zur der Organbildung des Embryo, Anat. Anz., 8:605.

Schechtman, A. M., 1939, Experiments on anus formation in a frog egg,

Proc. Soc. Exper. Biol. & Med., 41:48.

Schechtman, A. M., 1942, "The Mechanism of Amphibian Gastrulation:

I. Gastrulation-promoting Interactions Between Various Regions of an

Anuran Egg (Hyla regilla) ," University of California Publications

—

Zoology, vol. 51, pp. 1-39.

Schmalhausen, J. J., 1908, Die Entwickelung des Skelettes der hinteren

Extremitat der anuren Amphibien, Anat. Anz., 33:337.

Schultz, O. von, 1900, Ueber das erste Auftreten der bilateralen Symmetrie

im Verlauf der Entwicklung, Arch. mikr. Anat., 55:171.

, 1887, Untersuchungen iiber die Reifung und Befruchtung des

Amphibieneies, Ztschr. wiss. ZooL, 45:177.

Schwind, J. L., 1933, Tissue specificity at the time of metamorphosis of the

frog larvae, /. Exper. ZooL, 66:1.

Schwink, von, 1891, Untersuchungen liber die Entwicklung des Endothels

und der Blutkorperchen der Amphibien, Morphol. Jahrb., 17:288.

Shore, T. W., 1902, The development of the renal-portals and fate of the

posterior cardinal veins in the frog, J. Anat. & Physiol., 36:20.

Shumway, W., 1940, 1942, Stages in the normal development of Rana

ARTICLES 319

pipiens: I. External form, Anat. Rec, 78:139; II. Identification of the

stages from sectioned material, Anat. Rec, 83:309.

Silver, M. L., 1942, The motoneurons of the spinal cord of the frog, /.

Comp. Neurol., 77:1.

Spek, J., 1919, Studien iiber den Mechanismus des Gastruhiinvagination,

Biol. ZentralbL, 39:13.

Spemann, H., 1898, Ueber die erste Entwicklung der Tuba Eustachii und

des Kopfskelets von Rana temporaria, Zool. Jahrb., 11: 389.

Stenger, A., and H. A. Charipper, 1946, A study of adrenal cortical tissue

in Rana pipiens with special reference to metamorphosis, /. MorphoL,

78:27.

Stewart, S. G., 1927, The morphology of the frog's kidney, Anat. Rec,

36:259.

Stohr, Ph. von, 1895, Uber die Entwicklung der Hypochorda und des dor-

salen Pankreas bei Rana temporaria, Morphol. Jahrb., 23:123.

Suntzowa, W., 1931, Analyse der Gewebe beim Wachstum der Froschniere

in vitro. Arch. f. exper. Zellforsch., 10:178.

Swingle, W. W., 1926, The germ cells of Anurans: II. An embryological

study of sex differentiation in Rana catesbeana, J. Morphol. & Physiol.,

41:441.

Taylor, A. C, 1944, Selectivity of nerve fibers from the dorsal and ventral

roots in the development of the frog limb, /. Exper. Zool, 96:189.

, and J. J. Kollros, 1946, Stages in the normal development of Rana

pipiens larvae, Anat. Rec, 94:7.

Taylor, S. J., and E. H. Craigie, 1938, The vascularity of the hypophysis

of the frog (Rana pipiens), Anat. Rec, 71:277.

Tretjakoff, D., 1906, Die vordere Augenholfte des Frosches, Ztschr. wiss.

Zool., 80:327.

Tsui, C. L., 1931, Seasonal changes in the kidney of the frog, Contr. Biol.

Lab. Sci. Soc China, 7:239.

Ussoff, S. A. von, 1910, Stomodaeum-Ektochorda (das vordere Ende der

Chorda). Vergleichend-embryologische Studien des axialen Skelettes,

Anat. Anz., 35:168.

Villy, F., 1890, The development of the ear and accessory organs in the

common frog. Quart. J. Micr. Sci., 30:523.

Vintemberger, P., 1938, Sur les Resultats de la Transplantation d'Organin-

sateurs d'etendues differentes, chez Rana fusca, dans la Region Blasto-

porale ventrale, Comp. rend. Soc. de bioL, 127:433; Sur les Resultats de

la Transplantation d'Organinsateurs d'etendues differentes, chez Rana

fusca, dans la Region ventrale, de la voute blastuleene, Comp. rend. Soc.

de biol., 127:436.

Vogt, W., 1929, GestaUungsanalyse am Amphibienkeim mit ortlicher Vital-


farbung: II. Teil gastrulation und Mesodermbildung bei Urodelen und

Anuren, Arch. f. Entwcklngsmechn. d. Organ., 116:384.

Weber, A., 1908, Etude de la torsion de I'ebauche cardiaque chez Ranaesculenta, Bibliog. Anatomiqiie , 18:136.

Wetzel, G., 1908, Der Wassergeholt des fertigen Froscheies und der Me-chanismus der Bildung seiner Hiille im Eileiter, Arch. j. Entwcklngs-

mechn. d. Organ., 26:651.

Williams, S. C, 1930, Regeneration of peripheral nerves in Amphibia

studied with the aid of a vital stain, /. Exper. Zool., 57:145.

Wilson, C. B., 1896, The wrinkling of frog's eggs during segmentation,

Am. Naturalist, 30:761.

Wilson, H. v., 1902, Closure of the blastopore in the normally placed frog

egg, Anat. Anz., 20:123.

Witschi, E., 1929, Studies on sex differentiation and sex determination in

amphibians: I. Development and sexual differentiation of the gonads of

Rana sylvatica, J. Exper. Zool., 52:235.

Zwilling, E., 1941, The determination of the otic vesicle in Rana pipiens,

J. Exper. Zool., 86:333.

, 1940, An experimental analysis of the development of the anuran

olfactory organ, /. Exper. Zool., 84:291.

Index

Page numbers set in italics refer to illustrations in the text. Also consult the Glossary

for specialized terms.

Acetabulum, 218

Activation, 73, 77

Adelmann, H. B., 8

Adrenal cortex {syn. interrenal gland),

224, 225

Adrenal gland, 42, 222, 223, 224, 224

medulla of, 138, 189

medullary cords of, 224

Stilling cells of, 224

Adrenal tissue, 220

Aggregation, cell, 104

Albumen (see Jelly)

Amhlystoma punctatuin, 106, 107

Amphiaster, 87

Amphimixis (syn. syngamy), 73, 88

Amphioxus, 10, 76

Amplexus, 19, 41, 73, 74, 75, 222

Ampulla, 177

Anal plate (see under Plate)

Anaphase, 69

Annulus tympanicus (syn. tympanic ring),

178, 215, 216, 252

Anura, 17,41, 73,230

Anus (see also Proctodeum), 22, 23, 164,

205, 206

Aorta, descending (see Systemic trunk)

dorsal, 147, 149, 152, 168, 188, 198,

234, 236, 237, 238, 239, 240,

248,251

radix, 243

ventral (syn. truncus arteriosus), 234,

234,216,237,238,24^

Appendix, lateral, 779, 180, 182, 244, 253,

259

Arch, aortic, 133, 134, 148, 149, 150, 151,

152, 164, 195, 196, 236, 237,

239, 240, 245

branchial, 131, 132, 144, 237

hyoid, 131, 133, 144, 148, 150, 195

Arch—

(

Continued)

mandibular, 132, 133, 144, 148, 158,

165, 195

mesodermal, 144, 195, 196

neural, 213

systemic, 239

vertebral, 212

visceral, 131, 133, 148, 149, 158, 195,

195, 216, 251

Archenteron (syn. primitive gut; primary

embryonic cavity; intestine; gas-

trocoel) (see also Gut), 20,

113, 114, 115, 116, 118, 119,

120, 121,123,125, 133,135

Aristotle, 4

Aronson, L., 40

Arterial system (see under System)

Artery, basilar, 243

branchial, 158, 236, 237, 243, 248

caudal, 164, 187, 248

coeliac, 240

commissural, 243

cutaneous, 243

external carotid, 149, 238, 238, 239

gonadal, 240

iliac, 240

internal carotid (anterior cerebral),

148, 149, 164, 238 238, 243

intersegmental, 248

laryngeal, 240

lingual (visceral I), 243

lumbar, 240

mesenteric, 248

oesophageal, 240

ophthalmic, 239

palatine, 239

pulmo-cutaneous, 238, 239, 240, 243

pulmonary, 238, 243

renal, 240

spinal, 169

321

322 INDEX

Artery

—

(Continued)

vertebral, 239

visceral, 243

vitelline, 164

Articular cartilage bone, 216

Atlas (vertebra), 212

Atrio-ventricular valve, 242

Atrium, 145, 149, 152, 164, 168, 234, 234,

235, 247

Auditory capsule {see Otic capsule)

Auditory placode {syn. otic placode), 775,

777, 251

Auditory vesicle {see Otic vesicle)

Auricle, 234, 235, 242, 245, 252

Axis (embryonic), 21, 81, 121, 123

Axolotl, 34

Bvon Baer, 6, 1

1

laws of, 6, 12

Balfour's law, 88

Baltzer, F., 81

Basibranchial, 214

Basicranial cartilage {see ///it/er Cartilage)

Basicranial fontanelle {see under Fonta-

nelle)

Basilar chamber {see under Chamber)Basilar plate {see under Plate)

Bataillon, E., 8

Batuschek, 55

Bautzmann, 8

Behavior, reproductive, 40

Belt, marginal {see Germ ring, margin of)

Bernard, 55

Bidder's organ, 41, 229

Bile duct, 135, 204

Biogenesis, 11, 12

Bird, 10

Bladder, 41,205

Blastema, 21

1

Blastocoel, 3, 93, 94, 95, 104, 770. 114,

115, 119, 121. 725

Blastoderm, peacock, 13

Blastomere, 20, 90, 93

Blastopore, 770, 777, 119, 725, 127, 729,

7J5, 251

lips of, 101, 702. 705, 706, 709. Ill,

112, 113, 114, 115, 116

Blastula, 14, 20, 26, 93, 94, 103, 705, 107,

770, 776

hemisected, 94

Blastulation, 93, 94, 95

Blood, 98, 757, 246

Blood corpuscles, 228

Blood islands, 236, 240, 252

Blood vessel {see also specific vein or

artery), 133, 238

Blood vascular system {see Circulatory

system)

Body {see specific body)

Boell, E. G., 8

Bone(s) {see specific bones)

Bonnet. 5

Born, 5, 98

Bowman's capsule, 32, 39, 223, 225

Boyd, 64

Brachet. J.. 8

Brain. 70, 139, 163, 168, 191, 251, 259

ventricle of, 163, 164

Branchial cartilage bone, 276

Branchial chamber {see under Chamber)Branchial loop, afferent, 236

efferent, 236

Branchial vessel, 750, 752

Breeding, 19, 41,43,48

Bridge, completion, 775, 776, 119, 725

Bronchus, 203

Brow spot, 164

Brown, Robert. 7

Budding, 11

Bulbus arteriosus {syn. bulbus aortae),

145, 149, 152, 158, 234, 234,

238, 243, 245, 248

Butschli, 98

Canal(s), anterior semi-circular of the

ear {see under Ear)

central {see Neurocoel)

choanal (êe Choanal canal)

entrance, 253

horizontal semi-circular of the ear {see

under Ear)

inferior, 253

neural {see Neurocoel)

neurenteric {syn. chorda canal), 127,

729, 133, 135. 143, 206, 251

posterior semi-circular of the ear {see

under Ear)

semi-circular {see also specific canal),

176,251,259

spinal {see Neurocoel)

INDEX 323

Capillary loops, 236

Capsule (see specific capsule)

Carbon dioxide, 156

Carotid gland, 199, 239

Cartilage. 776, 214, 214

basibranchial, 217. 250

basicranial {syn. capula), 216

basihyal, 277

basihyoid, 277

ceratobranchial, 214, 217, 250

ceratohyal, 214, 216, 249

cranial, 213, 252

hyobranchial, 277

hyoid, 200,201. 2\6, 217,244

hypobranchial, 214, 217. 250

labial. 214. 249

Meckel's. 274, 216. 249

mental (syn. infra-rostral), 27-^, 249

mesotic. 276

occipital, 276

olfactory. 275

palato-quadrate, 214

trabecular (see Trabeculae)

Cartilage bones (see also specific cartilage

bones). 214

Cavity, glenoid, 218

inferior of olfactory organ (see under

Olfactory organ)

orbital, 775

pericardial, 747, 145, 151. 151. 158,

231.233, 2i7. 245, 247

peritoneal (i'eeCoelom)

pharyngeal, 194

pleuro-peritoneal, 232

primary genital, 228

segmentation (see Blastocoel)

superior of olfactory organ (see under

Olfactory organ)

Cell division (see also Cleavage, Mitosis),

3

Cells (see specific cells)

Centrosome, 34, 59, 80

Centrum, 213

Cerebelli valvulae, 166

Cerebellum, 40, 158, 163, 165, 167. 182,

190, 259

Cerebral hemisphere (see under Hemi-

sphere)

Cerebrum, 162

Chamber, anterior, 770

basilar, 260

Chamber

—

(Continued)

branchial (syn. opercular chamber), 23,

26,752, 157, 197,244,245

laryngeal, 213

pronephric, 221

Charipper, H. A., 202, 217, 224, 225

Chiasma (.<;ee also specific chiasma), 40

Child, C. M., 8

Chin, black, 31

Choana (see Nares, internal)

Choanal canal, 180. 182. 244, 253

Choanal fold, anterior, 253

Chondrocranium, 214. 215. 249

Chorda canal (see Canal, neurenteric)

Chorda-mesoderm, 770. 774, 775, 776

Choroid fissure, 166. 770, 777

Choroid knot. 770, 172

Choroid layer of the eye (see under Eye)

Choroid plexus, anterior. 142. 163, 163,

765, 190, 244, 259

posterior, 163, 167, 190, 245, 252

Christa ventralis, 253

Chromatin, 57. 60

Chromatophores, 138,254-258

Chromosome. 33, 34, 58, 60, 61, 62, 63,

64.66,69,71

Chromosome core. 65

Chromosome frame, 65, 65

Cilia, body (ectodermal, external), 26,

123,251,259

coelomic (peritoneal ) , 3 1 , 49, 5i, 54, 220

peritoneal (see Cilia, coelomic)

Circulation (see also specific circulation),

27, 253

Circulatory system (syn. blood vascular

system), 21, 98, 149, 236, 238,

240, 242, 243, 247, 252

Clavicle, 218

Cleavage (see also Mitosis, Cell division),

3, 19, 20, 26, 27, 67, 81, 87, 89,

91, 92, 92

furrow, 83, 88, 90,91

vertical. 89

gradient of, 95

holoblastic. 88, 93

horizontal, 92, 95

laws of, 88

meroblastic, 93

rate of. 19

Cleft, branchial. 705, 132, 134, 152, 159,

195. 196

324 INDEX

CMt— (Continued)

gill (see Cleft, branchial)

hyomandibular, 105

stomodeal {syn. stomodeal invagina-

tion), 23, 128, 130, 135, 142,

145, 148, 156, 195

visceral (syn. gill slits), 25, 133, 195,

196, 208

Cloaca, 42, 56, 135, 148, 164, 205, 207,

226, 245

Coating, surface, 104, 105, 107

Cochlea (see Lagena)

Coelenterata, 14

Coeloblastula, 93

Coelom (syn. splanchnocoel, peritoneal

cavity), 137, 145, 147, 151, 158,

219, 227, 228, 231, 245, 247

Collecting tubule, 36, 37, 40, 229

CoUiculus, inferior, 40

Colloid, 65

Columella (syn. plectrum), 178, 215, 252

Commissural fibers (see under Fibers)

Commissure, anterior, 162

anterior pallial, 162, 163, 164

posterior, 163, 164, 166

Concept, germ layer, 14

Conjunctiva, 175

Conklin, E. G., 8

Constriction, 109

Contraction stage (see under Stage)

Convergence, dorsal, 109

Cooper, Ruth, 253

Copenhaver, W. M., 8

Coracoid, 218

Cornea, 26, 28, 170, 192, 244, 252

Cornu trabeculae of olfactory organ (see

under Olfactory organ)

Corpora adiposa (see Fat bodies)

Corpora bigemina (see Optic lobe)

Corpuscle(s), 150

blood, 228

Malpighian (see Malpighian corpuscle)

renal (see Malpighian body)

Cranium, 214

placode of, 179, 181

Crepidula, 108

Crescent, gray, 19, 25, 27, 76, 81, 83, 84,

94, 96, 100, 109, 110, 111, 112

Crest segment (cranial), 182, 184, 191,

251

Croak, warning, 40

Croaking sound, 31, 43

Crura cerebri, 163, 166

Cutis of the eye (see under Eye)

Cutis plate (see Dermatome)Cuvier, 6

Cyst wall (see Theca interna)

Cytoplasm, 2, 3, 48, 58

DDalcq, A. W., 8, 98

D'Angelo, S. A., 202, 217

Davenport, 24

de Beer, G. R., 8

Delamination, 108

Dentary bones, 216

Dermal bones, 214, 216

Dermatome (syn. cutis plate), 147, 148,

186, 209, 211, 249

Dermis, 211

Detwiler, S. R., 8

Development, early, 1, 2, 3

embryonic, 4, 5, 19

rate of, 26

stages of, for Rana pipiens, 17, 28, 29

unfolding (see Epigenesis)

Diakenesis stage (see under Stage)

Diencephalon (syn. thalamencephalon.

"between brain"), 150, 152, 158,

164, 190,244

Differentiation, 101, 102, 103

cell (specialization). 15

Diocoel, 135, 141, 146, 150, 158, 162, 163,

164, 165, 168, 244

Diplotene, 34, 57

Disc, intervertebral (or ligament), 213

Division, equational, 33

post-reductional, 33

reductional, 33

Dorsal thickening (see Thickening, dor-

sal)

Driesch, H., 8

Duct, bile (see Bile duct)

endolymphatic, 152, 176, 176, 177, 177,

182, 192, 245, 252

hepatic, 204

hepato-pancreatic, 205

mesonephric (syn. Wolffian duct), 39,

40, 222, 122, 225, 226, 227, 228

Miillerian (see MUUerian duct)

pancreatic, 204

INDEX 325

Duct

—

(Continued)

pronephric (syn. segmental duct), 133,

147, 148, 152, 164, 219, 219,

222, 247, 252

thoracic (lymphatic system), 246

urogenital, 32, 39, 42, lid

Ductus arteriosus {syn. ductus Botalli),

238, 239, 240

Ductus Cuvieri, 232, 234, 238, 241, 245

Dumas, 6

Duodenum, 205, 206, 208

Duryee, W. R., 58-63, 66

Dye, vital (see Stain, vital)

EEar (see also Otic), 101, 102, 129. 175,

252

canal of, semi-circular, anterior, 776, 245

horizontal, 776,245

posterior, 776, 251, 259

operculum of, 178

space of, perilymph, 178, 252

Ectoblast, 97

Ectoderm, 3, 14, 20, 97, 98, 707, 107, 127

epithelial, 770, 114, 115, 124, 147

neural (i.e., nervous), 7/0, 775, 137,

7^7, 750

presumptive, 101

Egg (i.e., mature ovum), 1, 75, 19, 27, 44,

46, 49, 52, 55, 56, 57, 66, 68, 72,

72, 73, 88

fertilization of, 2

fertilized (syn. zygote), 1, 3, 4, 13, 14,

33, 82

frog's, 2, 49, 50

laying of, 19, 75

mackerel, 13

symmetry of, 82

telolecithal, 64, 73, 88

Ekman, 8

Elasticity, cellular, 104

Emboitement (see Preformationism)

Embryo, 1, 3, 22

rotation of, 26, 27, 102, 126

Embryologist, 9

comparative, 2

Embryology, 1, 4

cellular, 7

chemical, 8

comparative, 8

descriptive, 4

Embryology

—

(Continued)

dynamic, 1

1

experimental, 7, 8

Embryonic, extra, 13

Encasement theory (see Preformationism)

Enchytrea (syn. white worms), 30

Endocardium, 145, 150, 151, 153, 233,

237, 252

Endocrine glands (see also specific

glands), 157, 223, 260

Endoderm, 3, 14, 20, 97, 98, 707, 113, 775,

119, 193

presumptive, 101, 107

yolk, 148

Endolymphatic duct (see under Duct)

Endolymphatic tissue (see under Tissue)

Endothelial cells of the heart (see under

Heart

)

Enteron (see Gut or Archenteron)

Enzyme, 70, 155

Ependyma, 136, 167,769, 191

Epiblast, 3, 20, 21

Epiboly, 92, 107, 108, 111, 112, 113

Epicoracoid (syn. precoracoid), 218

Epidermis, 98, 707. 190

Epigenesis (syn. unfolding development),

5, 12, 13

Epimere (syn. segmental or vertebral

plate), 7i7,74.^, 146,209,279,247

Epiphyseal recess, 164

Epiphysis (becomes pineal body), 135,

141, 142, 158, 161, 762, 163,

164, 164, 765, 765, 190, 244, 251

Episternum, 218

Epithelioid body, 198,259

Epithelium, 21, 32, i6, J7, 98

germinal, 225, 229

Equatorial plate cells, 101

Ethmoid cartilage bones, 216, 249

Eustachian tube (see Tube, Eustachian)

Evolution, 12

Exoccipital cartilage bone, 216

Extension, 108

Eye (see also Optic, e.g. optic vesicle), 27,

707, 702, 729, 7^0, 765, 777,

174, 191,256, 257

cartilage of, 174

coat of, sclerotic, 174, 175, 192

cutis of, 775

layer of, choroid (syn. choroid coat),

774, 775, 192

326 INDEX

Eye— ( Continued)

nuclear, inner, 124

outer, 124

pigmented. 140, 146, 150, 166, 170,

171, 174, 175, 191, 244

pupil of, 171

vitreous humor of {syn. vitreous body),

172, 174, 175

Eyelid, lower, 175

upper, 175

Fabricius, 5

Fankhauser, G., 8

Fat bodies {syn. corpora adiposa). 18, 42,

43, 222, 229

Female, 18, 43, 54, 226

Femur, 218

Fertilization, 19, 20, 25, 33, 41, 51, 54, 73,

76,81

Fibers, commissural, 169

lens, 174

Fin, 136, 156

Fish, 10, 12

Fission, binary, 1

1

Fleming, 5, 7

Flexure, cranial, 138

Follicle (ovarian), 44, 45, 46, 48, 51, 52,

69

Follicle cells, 45, 46, 47, 57, 229

Follicular rupture {see Rupture, follicular

point of)

Fontanelle, basicranial, 216, 249

Food, 30, 157

Foot, 255, 256

Foramen magnum (skull), 216

Foramen ovale, 178

Forebrain {see Prosencephalon)

secondary (êe Telencephalon)

Foregut, 101, 102, 117, 135, 142, 143,

152, 168, 194

Fronto-parietal bones, 216

Funnels, peristomial (i.e., peritoneal), 49,

220

peritoneal {see also Nephrostome),

220, 223

Gall bladder, 135, 168, 204, 205, 245

Ganglia, acustico-facialis (VII-VIII),

181, 182, 184

Ganglia

—

{Continued)

cranial, 182, 252

dorsal root, 138, 169, 185, 188, 191, 252

spinal, 138, 158

Ganglion, gasserian, 184

glossopharyngeal, 181, 182

habenular, 163, 164

semi-lunar (of V), 181

sympathetic, 187, 188, 188, 191

trigeminal, 182

vagus {syn. X; pneumogastric), 181,

181, 182, 184

ventral root, 191

Ganglion cell layer (of eye), 174

Gastrea theory, 7

Gastrocoel {see Archenteron or Gut)

Gastrula, 3, 20, 27, 103,776

Gastrulation, 15, 20, 26, 97, 98, 700, 101,

102, 102, 107, 109, 770, 777

114,115, 116, 118,720

Gaup, 215

General biological supply house, 260

Genital ridge {see under Ridge)

Germ cell, 13

formation of, 17

pre- or primordial, 13, 33, 228, 228, 229

Germ layer(s), 12, 20, 97, 98, 99

derivatives of, 99

Germ ring, margin of {syn. marginal zone,

marginal belt), 94, 96, 98, 103,

105,770, 111, 112, 113,725

Germ wall {see Germ ring, margin of)

Germinal epithelium {see under Epithe-

lium)

Germinal vesicle {see Nucleus)

Gill anlage {syn. gill plate, gill bud), 27,

23, 26, 705, 729, 7iO, 132, 156,

251

Gill bud {see Gill anlage)

Gill chamber {see Chamber, branchial)

Gill circulation, 26, 28, 30, 252

Gill filaments, 245

Gill plate {see Gill anlage)

Gill rakers, 795, 245

Gill slits {see Cleft, visceral)

Gills, external, 27, 22, 23, 23, 25, 26, 132,

148, 149, 150, 155, 756, 156,

795, 197, 227, 237

internal, 23, 25, 26, 30, 197, 237, 245

Girdle, pectoral, 218

pelvic, 218

INDEX 327

Gland {see specific gland)

Glass, F. M.. 35

Glia ceils (see Neuroglia)

Glomeriilus,59,42, 2/9, 223

Glomus, 220, 243, 248, 252

Glottis, 134, 203

Goerttler. K., 8

Gonad, 13, 157, 224, 226, 227, 228, 230,

231, 245, 247, 259

Gonoducts, 225

Gradient {see Polarity)

Groove, hyomandibular {syn. hyoman-

dibular furrow), 131, 144

neural, 105, 123, 124, 137

visceral, 134, 143, 795, 195, 196

Growth, 15, 24, 33

Guinea pig, 13

Gut, post-anal, 206

primary embryonic cavity {see also In-

testine, Archenteron), 20, 23,

142, 147, 149, 157, 205

Guyenot, E., 8

HHadorn, E., 8

Haeckel, E., 7, 11

Hamburger, V., 8, 118

Harrison, R. G., 8

Hartsoeker, 6

Harvey, W., 5

Hatching, 21, 22, 23, 24, 26, 28, 155, 252

Head, 130

Head kidney {see Pronephros)

Heart, 25, 26, 30, 729, 135, 141, 145, 211,

232, 233, 234, 234, 235, 247,

251, 252, 253

endothelial cells of, 232

ventricle of, 149, 152, 164, 168, 234,

234. 236, 242, 248

Heart mesenchyme {see under Mesen-

chyme)

Heat, 25

Hemisphere, animal {syn. animal pole),

79, 44, 74, 76, 80, 92, 93, 94,

104,777, 112,776

cerebral, 40, 45, 163, 163, 190. 252

vegetal {syn. vegetal pole, yolk hemi-

sphere), 79, 45, 47, 76, 11, 94,

101, 104, 777, 112, 776

yolk {see Hemisphere, vegetal)

Hepatic duct {see under Duct)

Hepato-pancreatic duct {see under Duct)

Herbst. 8

Hertwig (Oscar, Paula, Richard), 6, 8

Hertwig's law, 88

Hibernation, 35, 37 , 56

Hindbrain {see Rhombencephalon)

Hindgut, 707, 777, 729, 133, 135, 142.

143, 148, 158, 206

His, W.. 8, 98

Holtfreter, J., 8, 48, 96, 98, 104, 105, 707,

117

Hormones, sex, 41, 43, 54, 226, 229

Horn(s), of spinal cord {see under Spinal

cord)

Huettner, 135, 143, 144, 195

Humerus, 218

Humor, vitreous, of eye {see under Eye)

Huxley, 5

Hyoid cornu, 217

Hyomandibular furrow {see Groove, hyo-

mandibular)

Hypomere {see Mesoderm, lateral plate)

Hypophyseal invagination {see under In-

vagination)

Hypophysis, 729, 755, 140, 141, 144, 148,

158, 162, 164, 165, 165, 765,

200,244,251

Hypothalamus, 40, 163

Ilium, 218

Inferior cavity (of olfactory organ) {see

under Cavity)

Infundibulum, 135, 140, 144, 150, 162,

163, 164, 765, 76S, 790, 200,

216,244,251

of oviduct {see Ostium)

Integument, 7S7

Intermediate cell mass {see Mesomere;

Nephrotome)Interrenal gland {see Adrenal cortex)

Interstitial tissue {see under Tissue)

Interstitial tissue of the testes {see under

Tissue)

Intestine, 205, 206, 208, 259

Invagination, 108

hypophyseal, 130

pseudo, 112

Involution, 702, 108, 777, 112

Iris, 172

Ischium, 218

328 INDEX

Jaws, 157, 159, 194, 214, 216, 259, 260

Jelly {syn. albuminous coating of the egg,

albumen), 22, 50, 54, 55, 56, 73,

76,76, 155

Jenkinson, 34, 46, 57, 69, 183, 229

Just, E. E., 8

KKidney, 18, 39, 42, 220, 220, 242

Kolliker, 7

Kollros, 254-258

Kopsch, 120

Korschelt, W., 8

Kowalsky, 7

Lacuna, 236

Lagena {syn. Cochlea), 177, 192, 260

Lamina terminalis, 141, 162, 162

Larva, 3, 22, 26, 158, 164, 252

Laryngeal chamber (see under Chamber)Lateral appendix (see Appendix, lateral)

Lateral groove of olfactory organ (see


Lateral line organ (see also Nerve, lateral

line), 179, 180, 181, 185, 251,

254, 260

Law(s) (see specific law)

Layers (see specific layer)

Leeuwenhoek, 5

Legs, 157

Lehmann, F. E., 8

Lens, 101, 102, 129, 140, 146, 152, 158,

164, 170, 171, 174, 175, 191,

244

Lens fibers (see under Fibers)

Lens placode, 172, 251

Lens vesicle, 140, 170, 111, 174, 182, 251

Leptotene stage (see under Stage)

Leuckart, 205

Lewis, 8

Lillie, F. and R., 8

Limb bud (syn. limb anlage), 22, 129,

253,254,255

Limbs, fore, 25, 26, 101, 129, 257, 258

hind, 26, 729,211,255-255

Lines, tension, 89

Lip(s), 21, 22, 26, 27, 157, 193, 244, 259

blastoporal, 101, 110

Lipoid substance, 62

Liver, 53, 98, 204, 208, 227, 242, 245Liver diverticulum (syn. Liver anlage),

135, 141, 143, 143, 144, 145,

147, 149, 158, 168, 204

Lobe (see specific lobe)

Loeb, J., 8, 73

Lumen of seminiferous tubule (see under

Seminiferous tubule)

Lung bud (syn. lung anlage), 134, 203,

205,243,245,253

Lungs, 25, 53, 98, 134, 168, 203, 205, 208,

227

Lymph heart, 246, 252

Lymph sac, 244, 245

Lymph space, 246

Lymphatic system (see under System)

MMacromeres, 91

Male, 18, 31,225

Malpighi, 5

Malpighian body (syn. renal corpuscle),

223

Malpighian corpuscle, 32, 37, 39, 40, 225

Mammal, 10

Mammillary recess, 163, 166

Mangold, O., 8

Mantle layer of spinal cord (see under

Spinal cord)

Maps, fate, 98, 101

Marginal layer of spinal cord (see under

Spinal cord)

Mark, 7

Matter, of spinal cord (see under Spinal

cord)

Maturation (syn. meiosis or meiotic divi-

sions), ii, 35, 35, 36, 37, 57, 65,

67, 71, 77

multiplication stage in, 33

Maturation spindle, 65, 71

Maxillary bone, 215, 216

Maxillary process (see under Process)

Mayer, B., 118

Meatus venosus, 233, 240

Meckel's cartilage (see ///zt/er Cartilage)

Medulla oblongata, 127, 158, 167, 259

Medullary cords of adrenal gland (see

under Adrenal glands)

Medullary plate (see under Plate)

Meiosis (see Maturation)

INDEX 329

Meiotic divisions {see Maturation)

Melanin, 64

Membrane, fertilization {see also Mem-brane, vitelline), 73, 77

nictitating, 175

nuclear. 65

pericardial, 153

tympanic {syn. tympanum), 31, 178,

185, 195, 257, 259

vitelline {see also Membrane, fertiliza-

tion), 45,5/, 55, 69, 73,81,83,

93

Membrane bones, 214

Mento-Meckelian cartilage bone, 216

Mesencephalon {syn. midbrain), 139, 140,

141, 158, 162, 163, 164, 166,

168, 190

Mesenchyme {syn. embryonic mesoderm)

{see also Mesoderm), 143, 144,

144, 145, 147, 158, 172, 177,

180, 184, 247,251,252heart, 135, 143, 144, 168

Mesentery, dorsal, 228, 232, 245, lAl

Mesocardium, dorsal, 757, 233, 235

ventral. 233, 245

Mesocoel {syn. aqueduct of Sylvius), 7i5,

144, 158. 162, 163, 167, 765, 190

Mesoderm {see also Mesenchyme), 3, 14,

20, 21, 97, 98, 77^, 725, 128,

141, 209, 247

embryonic {see Mesenchyme)gastral, 777

lateral plate {syn. hypomere, ventral

plate), 137, 145, 146, 147, 150,

186, 219, 231. 247, 251

peristomial, 77^, 775

somatic, 7i5, 750, 231

somite, 757, 144, 145, 147, 148

splanchnic, 135, 137, 144, 145, 147, 148,

750,231

Mesomere {syn. intermediate cell mass),

146, 147, 148, 149, 186, 218,

279, 247

Mesonephric duct {see under Duct)

Mesonephric vesicle, 221, 222

Mesonephros {syn. Wolffian body), 221,

227, 245, lAl, 253, 259, 260

Mesorchium, 31. 40, 42, 222, 228

Mesovarium, 43, 48, 228

Metamorphosis, 19, 22, 24, 25, 26, 30, 33,

157,254-258

Metamorphosis of spermatid {see under

Spermatid)

Metaphase, 69

Micromeres, 91

Midbrain {see Mesencephalon)

Midgut, 777, 729, 133, 137, 142, 143,

148, 152, 205

Mitosis (^yn. cell multiplication) {see also

Cleavage, Cell division), 3, 14,

33

Monro, foramen of, 162

Morgan, T. H., 8

Morphogenesis, 14, 15

Morphology, descriptive, 4

Mouth, 27, 22, 23, 25, 26, 28, 157, 755,

765, 184, 208, 252, 257, 260

Movement(s), morphogenetic, 700, 104

spawning, 40

Mucous glands, 259

MuUerian duct {syn. vestigial oviduct,

rudimentary male oviduct), 75,

41,^2,226Multiplication stage in maturation {see

under Stage)

Muscular response, 27, 26, 25, 30

Muscle, 98, 749, 155, 192,244,251,259

external rectus, 184

inferior obliquus, 183

rectus inferior, 183

medialis, 183

superior, 183

retractor bulbi, 184

superior oblique, 183

Myocardium. 745, 750, 75i, 233, 237, 252

Myocoel, 745, 209, 279, 231

Myotome {syn. muscle segment), 705,

132, 747, 149, 752. 756, 755,

209, 211, 279, 247, 251

NNares. external, 779, 750, 752, 192

internal {syn. choana), 779, 750, 192

Nasal bone, 275, 216

Nasal gland, 750

Needham, J., 8

Nephrocoel, 750, 219, 231

Nephrostome {see also Funnels, peri-

toneal), 219, 279, 223

Nephrotome {syn. nephrotomal mass, in-

termediate cell mass), 149, 218,

221

330 INDEX

Nerve, abducens, 184, 190, 259

afferent, 188

auditory {syn.otic), 177, 182, 184, 190,

251

brachial, 186

cranial, 146, 150, 158, 181, 182, 185,

190, 259

nucleus of, 40

efferent, 188

facial, 7S2, 184, 190,251

glossopharyngeal, 182, 184, 190

hyoid, 184

hypoglossal, 186,210

lateral line {see also Lateral line organ)

,

146, 147,152,158, 750,251

mandibular, 184

maxillary, 184

oculomotor (III), 183, 190

olfactory (I), 162, 163, 179, 180, 183,

190,259

ophthalmic, 184

optic, 140, 166, 172, 174, 175, 183, 190,

244, 252

otic (see Nerve, auditory)

palatine, 184

peripheral, 769

spinal, 146, 181, 182, 183, 185, 188, 191

sympathetic, 187

trigeminal, 752, 183, 190

trochlear, 166, 752, 183, 190

vagus, 752, 270

Nerve cord, 747, 752, 755, 163, 168, 185,

186, 190, 251

Nerve tract, efferent, 769

Nervous system, 98

central, 27, 123, 136, 190

peripheral, 169

sympathetic, 138, 188,252

Neural canal {see Neurocoel)

Neural crest, 124, 137, 138, 147, 169, 181,

183, 185, 756, 188,252

Neural fold, 27, 26, 707, 705, 123, 124,

126, 111, 132, 135, 137

Neural groove {see under Groove)

Neural plate {see Plate, medullary)

Neural tube, 26, 27, 124, 126, 129, 251

Neuroblast, 95, 137, 167, 182, 252,

259

Neurocoel {syn. neural canal; spinal canal;

central canal), 127, 729, 133,

135, 136, 137, 138, 141, 143,

144, 146, 147, 149, 152, 158,

162, 167, 765, 769, 756, 757,

191

Neurocranium {see also Skull), 249

NeurogHa cells {syn. glia cells), 167

Neuropore, 127, 138

Neurula, 27, 22, 127, 725, 132, 143

Neurulation, 26, 700, 103, 123, 136

Newport, 7

Nicholas, J. S.. 8, 98, 104

Nictitating membrane {see under Mem-brane)

Nile blue sulfate stain {see under Stain)

Nose {see Olfactory organ)

Notochord, 21, 98, 707, 702, 770, 113,

114, 115, 116, 117, 123, 124,

125, 135, 137, 141, 143, 143,

144, 146, 147, 149, 150, 152,

162

Nuclear layers of eye {see under Eye)

Nuclear membrane {see under Mem-brane)

Nucleic acid, desoxyribose, 2

ribo, 2

Nucleolus, 46, 47, 59, 60, 61, 64

Nucleus {syn. germinal vesicle), 2, 3, 34,

48, 59, 60, 65, 65, 66, 80

of cranial nerve, 40

sacs of, surface, 65

sperm of, 87

OOesophagus, 134, 148, 195, 204, 208, 259

plug of, 135, 168, 204, 251, 259

Olfactory bulb, 40

Olfactory capsule, 216

Olfactory cartilage, 275

Olfactory lobe, 162, 163, 179, 259

Olfactory organ, 27, 729, 178, 750, 192

cavity of, inferior, 750

superior, 750

cornu trabeculae of, 253

groove of, lateral, 750, 253

recess of, inferior, 253

posterior, 253

Olfactory pit, 27, 146, 148, 155, 156, 755.

164, 179, 180, 244

Olfactory placode, 142, 148, 158, 178. 750,

752, 251

Olfactory tube, 779, 259

Ontogeny, 1, 3

INDEX 331

Oocyte (syn. ovocyte), 33, 44, 45, 47, 48,

52, 59,61.(5.?, 64,66

Oogenesis, 33, 57

Oogonia, 33, 44, 44, 57

Opercular chamber (see Chamber, bran-

chial)

Operculum (syn. opercular fold), 21, 22,

23. 23, 25, 26, 29, 30, 131, 157,

178, 196, 197, 237, 258, 259

Operculum of ear (see under Ear)

Oppenheimer, J., 8

Optic chiasma, 70, 141, 162, 163, 164,

165, 166, 168, ni, 190

Optic cup, 140, 150, 152, 158, 164, 171,

173

Optic lobe (syn. corpora bigemina), 163,

166, 190, 259

Optic plate (see under Plate)

Optic protuberance, 130

Optic recess, 135, 141, 762. 163, 164, 168,

190

Optic stalk, 140, 142, 166, 770

Optic vesicle (syn. opticoel), 27, 705, 130,

131, 140, 142, 143, 144, 164,

766, 770, 170,251

Opticoel (see Optic vesicle)

Oral evagination, 135, 142, 143, 144, 162,

164

Oral membrane (see Plate, oral)

Oral plate (see under Plate)

Orbital bones, 216

Orbital cavity (see under Cavity)

Orbito-nasal foramen, 275

Organ(s) (see under specific organ)

anlagen of, 702, 729

formation of (syn. organogeny), 3, 15,

72^

of Jacobson, 179, 750

Organism, 1, 3

Organogeny (see Organ formation)

Osteoblasts, 213

Ostium (syn. infundibulum of the oviduct,

ostium tuba), 18, 49, 53, 54, 56,

226

Ostium tuba (see Ostium)

Otic capsule (syn. auditory capsule), 178,

7S2, 275. 216, 245,252

Otic placode (see Auditory placode)

Otic vesicle (syn. auditory vesicle), 144,

146, 150, 152, 158, 164, 177,

181, 190, 237

Ovary, 75, 43, 44, 48, 59

lobes of, 48

Oviduct, 18, 48, 49, 50, 51, 53, 54, 55, 56,

220, 226

infundibulum of (see Ostium)

rudimentary male (see Miillerian duct)

vestigial (see Miillerian duct)

Oviposition, 75

Ovist (see also Preformationism), 5, 12

Ovocyte (see Oocyte)

Ovulation, ^5, 5 1 , 52, 53, 56, 67, 65, 70

Ovum, mature (see Egg)

Oxygen, 25, 156

Pachytene, 34, 57

Palatine bones, 216

Palatoquadrate bone (syn. pterygoquad-

rate), 275, 216, 249

Pancreas, 98, 204, 205, 208, 259

Pancreatic duct (see under Duct)

Pander, 14

Papillae, horny rasping, 157

Parachordal bones, 214, 249

Parachordal plate (see under Plate)

Paraphysis, 164

Parasphenoid bones, 216

Parathyroid gland (syn. pseudo-thyroid

gland), 199, 208

Parmenter, 8

Pars distalis, 141

Pars intermedia, 141

Pars nervosa (syn. posterior pituitary

gland), 70, 141

Pars tuberalis, 141

Parthenogenesis, 5

Pasteels, J.. 8, 98,777. 725

Path, copulation, 78, 50, 57, 82, 87, 88

penetration, 78, 79, 80, 81, 88

pigment, 79

Patten, B. M., 8, 99

Penners, A., 8

Pericardial cavity (see under Cavity)

Pericardium, 145, 150, 151, 233, 237,

252

Perichondrium, 212

Perilymph space of the ear (see under

Ear)

Peritoneal cavity (see Coelom)

Peritoneal epithelium (see Peritoneum)

Peritoneal funnels (see under Funnels)

332 INDEX

Peritoneum (syn. peritoneal epithelium),

48, 53, 227, 232

Perivitelline space, 45, 77

Permeability, cell, 104

Pfliiger's law, 88

Pharyngeal cavity (see under Cavity)

Pharyngeal fold, 180, 253

Pharynx, 133, 135, 143, 144, 148, 149,

150, 151, 152, 158, 162, 164,

165, 168, 184, 194, 198, 200,

208, 237, 244

Pigment, 25, 47, 57, 80, 88, 95, 96, 115,

225

Pigmented layer of the eye {see under

Eye)

Pineal body or gland {see also Epiphysis),

164

Pit, polar body {see under Polar body)

Pituitary, anterior, 32, 37, 40, 48, 67, 70,

141, 163, 165, 244

posterior {see Pars nervosa)

Placode {see specific placode)

Planaria, 14

Plane, cleavage, 81

determination of. 87

median sagittal, 84, 87

penetration path, 82

Planes for sectioning, 9

Plasm, germ, 13

somato {seeSom&)

Plate, anal, 206

basilar, 214, lU, 249

cutis {see Dermatome)

ethmoid, 274. 216

gill {see Gill anlage)

hypobranchial, 244

intranasal {see Plate, ethmoid)

medullary {syn. neural plate), 26, 27,

105, 109, 114, 115, 123, 125,

126, 135, 137, 182

neural (see Plate, medullary)

optic, 130

oral {syn. oral membrane), 142, 168,

193

parachordal, 214, 259

segmental {see Epimere)

sense, 705, 127, 130,251

stapedial {see Stapes cartilage bone)

velar, 795, 245

ventral {see Mesoderm, lateral plate)

vertebral {see also Epimere)

Plectrum {see Columella)

Pleuro-peritoneal cavity {see under Cav-

ity)

Plexus, brachia, 186, 270

ischio-coccygeal, 188

lumbosacral, 186

sciatic, 186, 270

Point, sperm entrance, 80, 111

Polar body, 69, 71, 76, 78, 79, 81, 83

pit of, 77, 78

Polarity {syn. gradient), 64, 83, 85

cell, 104

Pole, animal {see Hemisphere, animal)

vegetal {see Hemisphere, vegetal)

Polyspermy, 74

Porter, K. R., 64, 79

Pouch, branchial {syn. gill pouch), 143,

144, 194, 795, 195

hyomandibular {see also Tube, Eusta-

chian)

visceral, 707, 133, 143, 144, 194, 195,

196

Precoracoid (êe Epicoracoid)

Predetermination, 1

Preformationism {syn. emboitement, en-

casement theory), 5, 6, 12, 13

Pre-maxillary bone, 275, 216

Preoptic area, 40

Pressure, 25

Prevost, 6

Primitive gut {see Archenteron; Intestine;

Gut)

Primitive streak, 134

Process, bony, 213

of skull {see under Skull)

maxillary, anterior, 275

posterior, 275

transverse, 213

Proctodeum {see also Anus), 98, 729, 132,

134, 135, 141, 143, 144, 145,

149, 158, 164, 189, 192, 193

Pronephric capsule, 220, 221

Pronephric chamber {see ///u/er Chamber)

Pronephric duct {see under Duct)

Pronephric region, 729, 130

Pronephric shelf, 219

Pronephric tubule, 148, 149, 152, 195

Pronephros {syn. head kidney), 705, 132,

750, 219, 220, 247, 251, 252, 259

Prootic cartilage bone, 216

Prootic foramen, 275

INDEX 333

Prophase, 34, 57

Prosencephalon {syn. forebrain), 139,

140, 161, 190, 251

Prosocoel, 144, 168

Pseudo-thyroid gland {see Parathyroid

gland)

Pterygoid bone, 275

Pterygoquadrate cartilage bone 'see

Palato-quadrate)

Pubis, 218

Pupil of the eye {see under Eye)

Quadrato-jugal bone, 215, 216

RRabl, 173

Ramus, dorsal, 191

spinal nerve (see Root, dorsal)

ventral, 191

Ramus communicans {syn. sympathetic

ramus), 169, 187, 188, 191

Rana catesbiana, 19, 24, 43, 66, 67

Rana clamitans, 67, 75

Rana pipiens, 17, 18, 19, 25, 32, 36, 43,

49, 58, 64, 65, 67, 75, 78, 79, 84,

102, 126, 143, 156, 157, 171,

214, 215, 254-258

Rana sylvatica, 31

Rana temporaria, 57 , 60, 229

Ratio, nucleo-cytoplasmic volume, 103,

104

Rawles, M., 8

Recapitulation, law of, 6, 12

Recess {see specific recess)

Rectum, 135, 143, 205, 206

Regeneration, 14

Regions, presumptive {syn. anlagen,

ebauche) {see also specific

regions), 14, 101

Release, 40

Remak, 7

Reproductive system {see under System)

Reptile, 10

Respiration. 237

Respiratory system {see under System)

Response, muscular, 21, 26, 28, 30

swimming, 40

Rete cords, 228

Retina, 140, 146, 158, 166, 170, 171, 174,

175, \9\, 244,251,259

Rhomaleum (grasshopper), 39

Rhombencephalon {syn. hindbrain). 140,

150. 152, 161, 164, 190

Rhombocoel {syn. fourth ventricle), 135,

141, 144, 150, 152, 158, 162,

167, 168, 237, 252

Rhumbler. 98

Ridge, genital, 227, 228, 229

mandibular, 128, 193

Ringer's solution, 50, 73

Rod, hypochordal {see Rod, subnoto-

chordal)

subnotochordal {syn. hypochordal

rod), 137, 141, 147, 151,206

Rods and cones, 171, 172, 251, 253, 259

Root, dorsal {syn. ramus of spinal nerve),

185,251

Rotation of embryo {see under Embryo)

Rotmann, E., 8

Roux, W., 8

Rudnick, D., 8

Rugh, R., 35

Rupture, point of follicular, 46, 47, 52

Saccule, 176, 177, 192, 245, 259, 260

Sach'slaw,88, 91

Sacral vertebrae, 212

Salamander, 61

Scale, phylogenetic, 14

Scapula, 218

Schechtmann, 104, 108, 118, 119

Schleiden, 7, 98

Schleip, W., 8

Schultz, 8

Schwann, 7

Scientist, definition of, 9

Sclerotic cartilage of the eye {see under

Eye)

Sclerotic coat of the eye {see under Eye)

Sclerotome, 147, 149, 186, 188, 210, 247,

251

Segment, muscle {see Myotome)Segmental duct (see Duct, pronephric)

Seminal vesicle, 41, 42, 226

Seminiferous tubule, 32, 36, 38, 39, 229

lumen of, 36, 37

Sense organs, 98

special, 170, 191

Septula {see Tissue, interstitial, of the

testes)

334 INDEX

Septum, interatrial, 235, 242, 252

interauricular {see Septum interatrial)

transversum, 232, 233

Sertoli cell, 36. 37, 38, 39, 40

Sex, characteristics of, female, 18, 43

male, 18, 31, 35

differentiation of, 228

Sex cell cord, 227, 231, 252

Sex cord, 230

Sheath, of vertebral column {see under

Vertebral column)

Shumway, W., 27, 28, 29

Sinus, superior, of the ear, 777

Sinus venosus, 145, 149, 158, 164, 234,

235, 238, 247

Siredon pisciformis, 173

Skeleton, appendicular, 218

visceral {syn. splanchnocranium), 21,

98, 143, 213, 214, 216, 249

Skin, 18, 159,211,257

Skull {see also Neocranium), 213

process of, anterior, 214

posterior, 214

Slit, gastrular, 112, 113,114,115,116,125

Soma {syn. somato-plasm, somatic tissue),

13, 14

Somatopleure, 137, 150, 151, 152, 181,

219

Somite {syn. segmental plate) {see also

Mesoderm, somite), 101, 102,

129, 137, 145, 146, 146, 147,

152, 158, 183, 209, 210, 211, 252

Spaces {see also specific space), 24

Spek, 98

Spemann, H., 8, 705, 113, 116,778

Spermatid, 33, 35, 36, 37, 38, 39

metamorphosis of, 33

Spermatocyte, 36, 37, 38

Spermatogenesis, 32, 33, 35, 35, 36, 37

Spermatogonium, 32, 33, 34, 35, 36, 37,

38,59

Spermatozoon, 5, 32, 33, 35, 35, 36, 37,

38, 38, 39, 39, 74, 78, 80, 83,

222

Spermist {see also Preformationism), 5,

6, 12

Spinal canal {see Neurocoel)

Spinal cord {see also Nerve cord), 163,

168, 185, 190,251

horn of, dorsal, 769

ventral, 769

Spinal cord

—

{Continued)

layer of, mantle, 191

marginal, 191

matter of, gray, 167, 757

white, 769, 187

Spindle, mitotic, 81

Spiracle, 21, 23, 23. 25, 26, 134, 157, 197.

245

Splanchnocoel {see Coelom)Splanchnocranium (see Skeleton, visceral)

Splanchnopleure, 137, 151, 152, 181, 219,

227

Spleen, 245, 247, 259

Spore formation, 1

1

Spratt, N. T., Jr., 8

Squamosal bone, 216

Stage, contraction {syn. synizesis), 33, 34,

57

diakenesis, 35

embryonic, 1

leptotene, 33, 34, 57

multiplication, in maturation, 33

pre-gastrulation, 103

synaptene, 33, 34, 57

Stain, Nile blue sulfate. 98

vital {syn. vital dye), 98, 700, 104

Stapes cartilage bone {syn. stapedial

plate), 216, 252

Starfish, 13

Stenger, A., 224, 225

Sternum, 218

Stilling cells of adrenal gland {see under

Adrenal gland)

Stomach, 53, 205, 205, 208, 227, 245, 259

Stomodeal cleft {see under Cleit)

Stomodeum, 98, 7iO, 144, 149, 189, 192,

193

Stroma, ovarian, 45, 57, 70

Structure(s), accessory, 41

epidermal, 98

Sucker, oral, 27, 22, 23, 101, 102, 105, 128,

129, 130, 150, 152, 156, 164,

244, 252, 254

Superior cavity of the olfactory organ {see


Surface sacs of the nucleus {see under

Nucleus)

Sutton, 7

Swammerdam, 5

Swimming response {see under Response)

Swingle, W. W., 8, 231

INDEX 335

Sylvius, aqueduct of {see Mesocoel)

Symmetry, bilateral, 82, 85

rotatory, 85

Synaptene stage {see under Stage)

Syngamy {see Amphimixis)

Synizesis stage {see Stage, contraction)

System, arterial, 236, 243, 248

circulatory {see Circulatory system)

lymphatic, 246, 249

nervous {see under Nervous system)

reproductive, 225

respiratory, 134, 157

urogenital, 98, 222

vascular {see Circulatory system)

venous, 240

Systemic trunk {syn. descending aorta),

239, 243

Tadpole, 19, 22, 23, 24, 25, 252

Tail bud, 21, 23, 26, 27, 129, 132, 134,

136, 144

Tail fin, 23, 26, 141, 148, 152, 157, 245,

253, 257

circulation of, 28, 30

Taylor, 254-258

Tectum, 40

Teeth, larval, 194, 217, 244, 254, 259

permanent, 29, 194, 217

Tegmentum, 40

Telencephalon {syn. secondary forebrain),

158, 161, 168, 190

Telocoel, 161, 180, 244

Temperature, 26, 30

Testis(es), 18, 31, 32, 37, 42, 220, 221,

231,242

interstitial tissue of {syn. septula), 32,

35,36

Thalamencephalon {see Diencephalon)

Thalamus, 40. 163

Theca externa, 43, 44, 45, 46, 47, 70

Theca interna {syn. cyst wall), 45, 46, 47,

69. 70

Theory {see specific theory)

Thickening, dorsal, 139, 165, 168

Thumb pad, 75, 31,55,43

Thymus, 198, 208, 259

Thyroid, 98, 135, 141. 145, 150, 158, 164,

168, 199, 200, 201, 202, 208,

277, 251, 259

Thyroid colloid, 207, 202

Tissue, adrenal {see Adrenal tissue)

connective, 98

endolymphatic, 90, 158

germinal, 14

interstitial, 229

of testes {syn. septula), 32, 35, 36

somatic (.see Soma)Tongue, 131, 184, 194, 200, 203, 260

Torus transversus, 7-^7, 161, 762, 168

Trabeculae {syn. trabecular cartilage),

27^,216,2^4,249

Trachea, 203, 208

Tract, alimentary, 98

Triturus torosus, 106

Truncus arteriosus {see Aorta, ventral)

Tube, Eustachian {see also Pouch, hyo-

mandibular), 134, 178, 195,252

neural {see Neural tube)

olfactory {see Olfactory tube)

Tuberculum posterius, 135, 139, 141 , 144,

162, 163, 165, 168

Tubule {see specific tubule)

Tunica albuginea, 32, 36

Twitty. V. C, 8

Tympanic membrane {see under Mem-brane

Tympanic ring {see Annulus tympanicus)

UUltimobranchial body {syn. supra-peri-

cardial or post-branchial body),

199. 208

Ureter, 225, 226

Uriniferous tubule, 32, 40, 223, 225

Urogenital ducts {see under Duct)

Urogenital system {see under System)

Urostyle, 270, 210, 212

Uterus, 49, 50, 56, 226

Utricle, 176, 177, 192, 259

Vas deferens {syn. Wolffian duct), 42,

225, 226, 245

Vasa efferentia, 222, 225, 229, 32, 39, 40,

42

Vascular system {see Circulatory system)

Vein, abdominal, 244

anterior cardinal {syn. jugular vein),

146, 150, 158, 164, 198, 238,

241,245,248

brachial, 241

336 INDEX

Vein

—

(Continued)

caudal, 164

common cardinal, 234, 241, 242, 249

cutaneous, 241

external jugular (syn. inferior jugular),

241, 242

femoral, 243

internal jugular (syn. superior jugular),

241, 242

hepatic, 238, 241, 242, 245, 249

hepatic portal (see also Vein, vitelline),

227, 241, 242

iliac, 243

medial cardinal, 242

pre-caval {see Vena cava, anterior)

posterior cardinal, 147, 158, 164, 222,

223, 238, 241, 242, 245, 249, 252

pulmonary, 235, 238, 242, 252

renal, 222, 242

renal portal (syn. subcardinal vein),

222, 242, 243, 249

sciatic, 243

subcardinal (see Vein, renal portal)

subclavian, 2i^,25S, 241

vitelline (see also hepatic portal vein),

145, 149, 152, 158, 164, 234,

235,238,240,242,251

Velar plate (see under Plate)

Vena cava, anterior (syn. superior vena

cava, pre-caval vein), 241, 242,

242

posterior (syn. inferior vena cava, post-

caval vein), 222, 228. 238, 241,

242, 259

Venous system (see under System)

Ventricle of heart (see under Heart)

of brain (see under Brain)

Ventriculus impar (syn. third or internal

brain ventricle), 162

Vertebrae (see specific vertebrae)

Vertebral column, 21, 143, 210, 210, 212,

260

sheath of, elastic, 212

fibrous, 212

skeletogenous, 212

Vertebral plate (see under Plate)

Vesicle (see specific vesicle)

Vessel (see specific vessel)

Vintemberger, 98

Virchow, 5, 7

Vitelline membrane (see under Mem-brane)

Vogt, W., 8, 98, 101, 108, 109

Volume relation, nucleo-cytoplasmic, 3

Vomer bones, 216

WWall (see specific wall)

Water, 76

Weismann, A., 8, 13

Weiss, P., 8

White worms (see Enchytrea)

Whitman, 7

Willier, B. H., 8

Wilson, E. B., 8

Wolffian body (see Mesonephros)

Wolffian duct (see Duct, mesonephric;

Vas deferens)

XXiphisternum, 218

Yolk, 21, 59, 80, 114, 116, 126, 129, 135,

137, 141, 143, 147, 152, 164,

168

Yolk nucleus, 61

Yolk platelets, 60, 62

Yolk plug, 26, 102, 105, 110, 112, 114,

115, 117, 121, 125, 126

Ziegler, 215

Zone, intermediate, 101

marginal (syn. marginal cells, germ

wall) (see Germ ring, margin

of)

Zygote (see Egg, fertilized)

'^. ^;:

Documents

Frog Its Reproduct 00 r Ugh